Combined with existing masking, provides protection against speculative
OOB accesses.

R=clemensh@chromium.org

Bug: chromium:798964
Change-Id: Ib7cdc8bccc6d22b8b45896c63f69cb647deba383
Reviewed-on: https://chromium-review.googlesource.com/856980
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50448}