math/big: use clearer loop bounds check elimination

Checking that the lengths are equal and panicking teaches the compiler
that it can assume “i in range for z” implies “i in range for x”, letting us
simplify the actual loops a bit.

It also turns up a few places in math/big that were playing maybe a little
too fast and loose with slice lengths. Update those to explicitly set all the
input slices to the same length.

These speedups are basically irrelevant, since they only happen
in real code if people are compiling with -tags math_big_pure_go.
But at least the code is clearer.

benchmark \ system                   c3h88    c2s16       s7      386   s7-386   c4as16      mac      arm  loong64  ppc64le  riscv64    s390x
AddVV/words=1/impl=go                    ~  +11.20%   +5.11%   -7.67%   -7.77%   +1.90%  +10.76%  -33.22%        ~  +10.98%        ~   +6.60%
AddVV/words=10/impl=go             -22.12%  -13.48%  -10.37%  -17.95%  -18.07%  -24.58%  -22.04%  -29.95%  -14.22%        ~   -6.33%   +3.66%
AddVV/words=16/impl=go              -9.75%  -13.73%        ~  -21.90%  -18.66%  -30.03%  -20.45%  -28.09%  -17.33%   -7.15%   -8.96%  +12.55%
AddVV/words=100/impl=go             -5.91%   -1.02%        ~  -29.23%  -22.18%  -25.62%   -6.49%  -23.59%  -22.31%   -1.88%  -14.13%   +9.23%
AddVV/words=1000/impl=go            -0.52%   -0.19%   -3.58%  -33.89%  -23.46%  -22.46%        ~  -24.00%  -24.73%   +0.93%  -15.79%  +12.32%
AddVV/words=10000/impl=go                ~        ~        ~  -33.79%  -23.72%  -23.79%   -5.98%  -23.92%        ~   +0.78%  -15.45%   +8.59%
AddVV/words=100000/impl=go               ~        ~        ~  -33.90%  -24.25%  -22.82%   -4.09%  -24.63%        ~   +1.00%  -13.56%        ~
SubVV/words=1/impl=go                    ~  +11.64%  +14.05%        ~   -4.07%        ~  +10.79%  -33.69%        ~        ~   +3.89%  +12.33%
SubVV/words=10/impl=go             -10.31%  -14.09%   -7.38%  +13.76%  -13.25%  -18.05%  -20.08%  -24.97%  -14.15%  +10.13%   -0.97%   -2.51%
SubVV/words=16/impl=go              -8.06%  -13.73%   -5.70%  +17.00%  -12.83%  -23.76%  -17.52%  -25.25%  -17.30%   -2.80%   -4.96%  -18.25%
SubVV/words=100/impl=go             -9.22%   -1.30%   -2.76%  +20.88%  -14.35%  -15.29%   -8.49%  -19.64%  -22.31%   -0.68%  -14.30%   -9.04%
SubVV/words=1000/impl=go            -0.60%        ~   -3.43%  +23.08%  -16.14%  -11.96%        ~  -28.52%  -24.73%        ~  -15.95%   -9.91%
SubVV/words=10000/impl=go                ~        ~        ~  +26.01%  -15.24%  -11.92%        ~  -28.26%   +4.25%        ~  -15.42%   -5.95%
SubVV/words=100000/impl=go               ~        ~        ~  +25.71%  -15.83%  -12.13%        ~  -27.88%   -1.27%        ~  -13.57%   -6.72%
LshVU/words=1/impl=go               +0.56%   +0.36%        ~        ~        ~        ~        ~        ~        ~        ~        ~        ~
LshVU/words=10/impl=go             +13.37%   +4.63%        ~        ~        ~        ~        ~   -2.90%        ~        ~        ~        ~
LshVU/words=16/impl=go             +22.83%   +6.47%        ~        ~        ~        ~        ~        ~   +0.80%        ~        ~   +5.88%
LshVU/words=100/impl=go             +7.56%  +13.95%        ~        ~        ~        ~        ~        ~   +0.33%   -2.50%        ~        ~
LshVU/words=1000/impl=go            +0.64%  +17.92%        ~        ~        ~        ~        ~   -6.52%        ~   -2.58%        ~        ~
LshVU/words=10000/impl=go                ~  +17.60%        ~        ~        ~        ~        ~   -6.64%   -6.22%   -1.40%        ~        ~
LshVU/words=100000/impl=go               ~  +14.57%        ~        ~        ~        ~        ~        ~   -5.47%        ~        ~        ~
RshVU/words=1/impl=go                    ~        ~        ~        ~        ~        ~        ~        ~        ~        ~        ~   +2.72%
RshVU/words=10/impl=go                   ~        ~        ~        ~        ~        ~        ~   +2.50%        ~        ~        ~        ~
RshVU/words=16/impl=go                   ~   +0.53%        ~        ~        ~        ~        ~   +3.82%        ~        ~        ~        ~
RshVU/words=100/impl=go                  ~        ~        ~        ~        ~        ~        ~   +6.18%        ~        ~        ~        ~
RshVU/words=1000/impl=go                 ~        ~        ~        ~        ~        ~        ~   +7.00%        ~        ~        ~        ~
RshVU/words=10000/impl=go                ~        ~        ~        ~        ~        ~        ~        ~        ~        ~        ~        ~
RshVU/words=100000/impl=go               ~        ~        ~        ~        ~        ~        ~   +7.05%        ~        ~        ~        ~
MulAddVWW/words=1/impl=go          -10.34%   +4.43%  +10.62%   -1.62%   -4.74%   -2.86%  +11.75%        ~   -8.00%   +8.89%   +3.87%        ~
MulAddVWW/words=10/impl=go          -1.61%   -5.87%        ~   -8.30%   -4.55%   +0.87%        ~   -5.28%  -20.82%        ~        ~   -2.32%
MulAddVWW/words=16/impl=go          -2.96%   -5.28%        ~   -9.22%   -5.28%        ~        ~   -3.74%  -19.52%   -1.48%   -2.53%   -9.52%
MulAddVWW/words=100/impl=go         -3.89%   -7.53%   +1.93%  -10.49%   -4.87%   -8.27%        ~        ~   -0.65%   -0.61%   -7.59%  -20.61%
MulAddVWW/words=1000/impl=go        -0.45%   -3.91%   +4.54%  -11.46%   -4.69%   -8.53%        ~        ~   -0.05%        ~   -8.88%  -19.77%
MulAddVWW/words=10000/impl=go            ~   -3.30%   +4.10%  -11.34%   -4.10%   -9.43%        ~   -0.61%        ~   -0.55%   -8.21%  -18.48%
MulAddVWW/words=100000/impl=go      -0.30%   -3.03%   +4.31%  -11.55%   -4.41%   -9.74%        ~   -0.75%   +0.63%        ~   -7.80%  -19.82%
AddMulVVWW/words=1/impl=go               ~  +13.09%  +12.50%   -7.05%  -10.41%   +2.53%  +13.32%   -3.49%        ~  +15.56%   +3.62%        ~
AddMulVVWW/words=10/impl=go        -15.96%   -9.06%   -5.06%  -14.56%  -11.83%   -5.44%  -26.30%  -14.23%  -11.44%   -1.79%   -5.93%   -6.60%
AddMulVVWW/words=16/impl=go        -19.05%  -12.43%   -6.19%  -14.24%  -12.67%   -8.65%  -18.64%  -16.56%  -10.64%   -3.00%   -7.61%  -12.80%
AddMulVVWW/words=100/impl=go       -22.13%  -16.59%  -13.04%  -13.79%  -11.46%  -12.01%   -6.46%  -21.80%   -5.08%   -3.13%  -13.60%  -22.53%
AddMulVVWW/words=1000/impl=go      -17.07%  -17.05%  -14.08%  -13.59%  -12.13%  -11.21%        ~  -22.81%   -4.27%   -1.27%  -16.35%  -23.47%
AddMulVVWW/words=10000/impl=go     -15.03%  -16.78%  -14.23%  -13.86%  -11.84%  -11.69%        ~  -22.75%  -13.39%   -1.10%  -14.37%  -22.01%
AddMulVVWW/words=100000/impl=go    -13.70%  -14.90%  -14.26%  -13.55%  -12.04%  -11.63%        ~  -22.61%        ~   -2.53%  -10.42%  -23.16%

Change-Id: Ic6f64344484a762b818c7090d1396afceb638607
Reviewed-on: https://go-review.googlesource.com/c/go/+/665155


Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>