If the input memory index into an atomic operation is a constant, we can
statically check if the {index+offset} is aligned.

R=herhut@chromium.org

Change-Id: Ia830d7c7df10d50ed4ee3382acfef776306f249c
Reviewed-on: https://chromium-review.googlesource.com/c/1362872
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58043}