internal/runtime/maps: eliminate a load from the hot path

internal/runtime/maps: eliminate a load from the hot path

typ.Group.Size involves two loads.

Instead cache GroupSize as a separate fields of the map type
so we can get to it in just one load.

Change-Id: I10ffdce1c7f75dcf448da14040fda78f0d75fd1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/627716
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>