internal/runtime/maps: optimize small map lookups with int keys
Load the field we need from the type once outside the search loop.
Get rid of the multiply to compute the slot position. Instead compute
the slot position incrementally using addition.
Move the hashing later in access2.
Based on khr@'s CL 618959.
For #54766.
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: Id11b5479fa5bc0130a1d8d9e664d0206d24942ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/620217 Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>