internal/runtime/maps: shift optimizations

internal/runtime/maps: shift optimizations

Masking the shift lets the compiler elide a few instructions for
handling a shift of > 63 bits.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: I669fe01caa1de1b8521f1f56b6906f3e9066a39b
Reviewed-on: https://go-review.googlesource.com/c/go/+/611190
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>