internal/runtime/maps: simplify small group lookup

internal/runtime/maps: simplify small group lookup

We don't really need the index of the slot we're looking at.
Just keep looking until there are no more filled slots.

This particularly helps when there are only a few filled entries
(packed at the bottom), and we're looking for something that isn't
there. We exit earlier than we would otherwise.

goos: darwin
goarch: arm64
pkg: runtime
cpu: Apple M2 Ultra
                                                 │  baseline   │              experiment               │
                                                 │   sec/op    │   sec/op     vs base                  │
MapSmallAccessHit/Key=int64/Elem=int64/len=1-24    2.759n ± 0%   2.779n ± 2%        ~ (p=0.055 n=10)
MapSmallAccessHit/Key=int64/Elem=int64/len=2-24    2.862n ± 1%   2.922n ± 1%   +2.08% (p=0.000 n=10)
MapSmallAccessHit/Key=int64/Elem=int64/len=3-24    3.003n ± 0%   3.061n ± 1%   +1.91% (p=0.000 n=10)
MapSmallAccessHit/Key=int64/Elem=int64/len=4-24    3.170n ± 1%   3.188n ± 1%   +0.57% (p=0.030 n=10)
MapSmallAccessHit/Key=int64/Elem=int64/len=5-24    3.387n ± 1%   3.391n ± 1%        ~ (p=0.362 n=10)
MapSmallAccessHit/Key=int64/Elem=int64/len=6-24    3.601n ± 1%   3.584n ± 0%   -0.49% (p=0.009 n=10)
MapSmallAccessHit/Key=int64/Elem=int64/len=7-24    3.785n ± 1%   3.778n ± 3%        ~ (p=0.987 n=10)
MapSmallAccessHit/Key=int64/Elem=int64/len=8-24    3.960n ± 1%   3.946n ± 1%        ~ (p=0.256 n=10)
MapSmallAccessMiss/Key=int64/Elem=int64/len=0-24   2.004n ± 1%
MapSmallAccessMiss/Key=int64/Elem=int64/len=1-24   5.145n ± 1%   2.411n ± 1%  -53.14% (p=0.000 n=10)
MapSmallAccessMiss/Key=int64/Elem=int64/len=2-24   5.128n ± 0%   3.313n ± 1%  -35.40% (p=0.000 n=10)
MapSmallAccessMiss/Key=int64/Elem=int64/len=3-24   5.159n ± 1%   3.690n ± 1%  -28.48% (p=0.000 n=10)
MapSmallAccessMiss/Key=int64/Elem=int64/len=4-24   5.117n ± 1%   4.466n ± 6%  -12.73% (p=0.000 n=10)
MapSmallAccessMiss/Key=int64/Elem=int64/len=5-24   5.115n ± 1%   4.308n ± 1%  -15.79% (p=0.000 n=10)
MapSmallAccessMiss/Key=int64/Elem=int64/len=6-24   5.111n ± 1%   4.538n ± 2%  -11.19% (p=0.000 n=10)
MapSmallAccessMiss/Key=int64/Elem=int64/len=7-24   4.896n ± 4%   4.831n ± 1%   -1.33% (p=0.001 n=10)
MapSmallAccessMiss/Key=int64/Elem=int64/len=8-24   4.905n ± 1%   5.121n ± 1%   +4.40% (p=0.000 n=10)
geomean                                            3.917n        3.631n       -11.11%

Change-Id: Ife26ac457a513af24fa0921b839ee6cd5fed6fba
Reviewed-on: https://go-review.googlesource.com/c/go/+/627717
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>