runtime, internal/runtime/maps: speed-up empty/zero map lookups

This lets the inliner do a better job optimizing the mapKeyError call.

goos: linux
goarch: amd64
pkg: runtime
cpu: AMD Ryzen 5 4600G with Radeon Graphics
                                 │ /tmp/before2 │             /tmp/after3             │
                                 │    sec/op    │   sec/op     vs base                │
MapAccessZero/Key=int64-12          1.875n ± 0%   1.875n ± 0%        ~ (p=0.506 n=25)
MapAccessZero/Key=int32-12          1.875n ± 0%   1.875n ± 0%        ~ (p=0.082 n=25)
MapAccessZero/Key=string-12         1.902n ± 1%   1.902n ± 1%        ~ (p=0.256 n=25)
MapAccessZero/Key=mediumType-12     2.816n ± 0%   1.958n ± 0%  -30.47% (p=0.000 n=25)
MapAccessZero/Key=bigType-12        2.815n ± 0%   1.935n ± 0%  -31.26% (p=0.000 n=25)
MapAccessEmpty/Key=int64-12         1.942n ± 0%   2.109n ± 0%   +8.60% (p=0.000 n=25)
MapAccessEmpty/Key=int32-12         2.110n ± 0%   1.940n ± 0%   -8.06% (p=0.000 n=25)
MapAccessEmpty/Key=string-12        2.024n ± 0%   2.109n ± 0%   +4.20% (p=0.000 n=25)
MapAccessEmpty/Key=mediumType-12    3.157n ± 0%   2.344n ± 0%  -25.75% (p=0.000 n=25)
MapAccessEmpty/Key=bigType-12       3.054n ± 0%   2.115n ± 0%  -30.75% (p=0.000 n=25)
geomean                             2.305n        2.011n       -12.75%

Change-Id: Iee83930884dc4c8a791a711aa189a1c93b68d536
Reviewed-on: https://go-review.googlesource.com/c/go/+/663495


Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>