math/big: replace division with multiplication by reciprocal word

Division is much slower than multiplication. And the method of using
multiplication by multiplying reciprocal and replacing division with it
can increase the speed of divWVW algorithm by three times,and at the
same time increase the speed of nats division.

The benchmark test on arm64 is as follows:
name                     old time/op    new time/op    delta
DivWVW/1-4                 13.1ns ± 4%    13.3ns ± 4%      ~     (p=0.444 n=5+5)
DivWVW/2-4                 48.6ns ± 1%    51.2ns ± 2%    +5.39%  (p=0.008 n=5+5)
DivWVW/3-4                 82.0ns ± 1%    69.7ns ± 1%   -15.03%  (p=0.008 n=5+5)
DivWVW/4-4                  116ns ± 1%      71ns ± 2%   -38.88%  (p=0.008 n=5+5)
DivWVW/5-4                  152ns ± 1%      84ns ± 4%   -44.70%  (p=0.008 n=5+5)
DivWVW/10-4                 319ns ± 1%     155ns ± 4%   -51.50%  (p=0.008 n=5+5)
DivWVW/100-4               3.44µs ± 3%    1.30µs ± 8%   -62.30%  (p=0.008 n=5+5)
DivWVW/1000-4              33.8µs ± 0%    10.9µs ± 1%   -67.74%  (p=0.008 n=5+5)
DivWVW/10000-4              343µs ± 4%     111µs ± 5%   -67.63%  (p=0.008 n=5+5)
DivWVW/100000-4            3.35ms ± 1%    1.25ms ± 3%   -62.79%  (p=0.008 n=5+5)
QuoRem-4                   3.08µs ± 2%    2.21µs ± 4%   -28.40%  (p=0.008 n=5+5)
ModSqrt225_Tonelli-4        444µs ± 2%     457µs ± 3%      ~     (p=0.095 n=5+5)
ModSqrt225_3Mod4-4          136µs ± 1%     138µs ± 3%      ~     (p=0.151 n=5+5)
ModSqrt231_Tonelli-4        473µs ± 3%     483µs ± 4%      ~     (p=0.548 n=5+5)
ModSqrt231_5Mod8-4          164µs ± 9%     169µs ±12%      ~     (p=0.421 n=5+5)
Sqrt-4                     36.8µs ± 1%    28.6µs ± 0%   -22.17%  (p=0.016 n=5+4)
Div/20/10-4                50.0ns ± 3%    51.3ns ± 6%      ~     (p=0.238 n=5+5)
Div/40/20-4                49.8ns ± 2%    51.3ns ± 6%      ~     (p=0.222 n=5+5)
Div/100/50-4               85.8ns ± 4%    86.5ns ± 5%	   ~     (p=0.246 n=5+5)
Div/200/100-4               335ns ± 3%     296ns ± 2%   -11.60%  (p=0.008 n=5+5)
Div/400/200-4               442ns ± 2%     359ns ± 5%   -18.81%  (p=0.008 n=5+5)
Div/1000/500-4              858ns ± 3%     643ns ± 6%   -25.06%  (p=0.008 n=5+5)
Div/2000/1000-4            1.70µs ± 3%    1.28µs ± 4%   -24.80%  (p=0.008 n=5+5)
Div/20000/10000-4          45.0µs ± 5%    41.8µs ± 4%    -7.17%  (p=0.016 n=5+5)
Div/200000/100000-4        1.51ms ± 7%    1.43ms ± 3%    -5.42%  (p=0.016 n=5+5)
Div/2000000/1000000-4      57.6ms ± 4%    57.5ms ± 3%      ~     (p=1.000 n=5+5)
Div/20000000/10000000-4     2.08s ± 3%     2.04s ± 1%      ~     (p=0.095 n=5+5)

name                     old speed      new speed      delta
DivWVW/1-4               4.87GB/s ± 4%  4.80GB/s ± 4%      ~     (p=0.310 n=5+5)
DivWVW/2-4               2.63GB/s ± 1%  2.50GB/s ± 2%    -5.07%  (p=0.008 n=5+5)
DivWVW/3-4               2.34GB/s ± 1%  2.76GB/s ± 1%   +17.70%  (p=0.008 n=5+5)
DivWVW/4-4               2.21GB/s ± 1%  3.61GB/s ± 2%   +63.42%  (p=0.008 n=5+5)
DivWVW/5-4               2.10GB/s ± 2%  3.81GB/s ± 4%   +80.89%  (p=0.008 n=5+5)
DivWVW/10-4              2.01GB/s ± 0%  4.13GB/s ± 4%  +105.91%  (p=0.008 n=5+5)
DivWVW/100-4             1.86GB/s ± 2%  4.95GB/s ± 7%  +165.63%  (p=0.008 n=5+5)
DivWVW/1000-4            1.89GB/s ± 0%  5.86GB/s ± 1%  +209.96%  (p=0.008 n=5+5)
DivWVW/10000-4           1.87GB/s ± 4%  5.76GB/s ± 5%  +208.96%  (p=0.008 n=5+5)
DivWVW/100000-4          1.91GB/s ± 1%  5.14GB/s ± 3%  +168.85%  (p=0.008 n=5+5)

Change-Id: I049f1196562b20800e6ef8a6493fd147f93ad830
Reviewed-on: https://go-review.googlesource.com/c/go/+/250417


Trust: Giovanni Bajo <rasky@develer.com>
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>