internal/runtime/maps: use simpler calculation for slot element
This reduces the adds required at the return point from 3 to 1.
(The multiply inside g.elem() does get CSE'd with the one inside
g.key(), but the rest of the adds don't.)
Instead, compute the element as just a fixed offset from the key.
Change-Id: Ia4d7664efafcdca5e9daeb77d270651bb186232c
Reviewed-on: https://go-review.googlesource.com/c/go/+/629535 Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Pratt <mpratt@google.com>