When loading a known value from a JSArray with a copy-on-write backing
store, we don't need to actually do a map check on the JSArray, but just
check that the backing store didn't change in the meantime.

R=jarin@chromium.org

Bug: v8:6816, v8:6815
Change-Id: I6764f3b8af7d4c17b9f6d2396555b584eae08176
Reviewed-on: https://chromium-review.googlesource.com/691721Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48218}