We can use the known ElementsKind to improve typing on the receiver
element load. We can allow multiple maps, as long as they have the
same ElementsKind.

Bug: v8:6896
Change-Id: Ida7df943f7d315454b58bcf4e0bbd2346406c488
Reviewed-on: https://chromium-review.googlesource.com/704921
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48367}