When accessing elements of a global (constant) JSArray, whose backing
store is copy-on-write, we can just constant-fold the value and insert
a check that the backing store stays the same.

Bug: v8:6816, v8:6815
Change-Id: I090bcec7b1ce72a1f9ed8625680ed91e8c67f27f
Reviewed-on: https://chromium-review.googlesource.com/662757Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47963}