It causes a significant regression in TypedArray benchmarks. Thinking it
through, it is actually not necessary for the JavaScript memory model
either. I originally added it to ensure that reads/writes are not elided
or duplicated, but there is no guarantee of this behavior for non-atomic
writes in the model.

BUG=chromium:763814
R=clemensh@chromium.org

Change-Id: Ib03d2e2e77a846d4b9e84eebc7f8fbf861f8fd7c
Reviewed-on: https://chromium-review.googlesource.com/661192Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47957}