By using 64-bit comparison we make sure that there will be no out of
memory accesses even if there are stale values in the high word of a
register.

R=titzer@chromium.org

Change-Id: I2627b15e1598f35cc480d7028031e8de405164ea
Reviewed-on: https://chromium-review.googlesource.com/721323Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48604}