MapRef::AsElementsKind can now concurrently walk transitions to find a
map of the requested elements kind.

Note this implementation is still less powerful than what we had before
crrev.com/c/3021175, since we never allocate new maps. When the
transition walk fails to find an appropriate map, we bail out.

I don't expect this to be a problem - when optimizing, the code has
already run multiple times and transitioned maps should exist.

Bug: v8:7790, v8:11988
Change-Id: Ic767b40c29bb86f7c4167097c76c5417985420fb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3086471
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76216}