Call to ReduceKeyedLoadFromHeapConstant got lost in rebasing,
as did the kHas check in ReduceElementAccessOnString. Added
some tests to ensure both cases are covered.

Change-Id: I8d6992c33315436b6228471b9bc57e3b267ad09f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1508837Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Matt Gardner <magardn@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#60132}