This CL adds a helper function that simplifies a bounds check pattern
that appears repeatedly in the code.

R=clemensh@chromium.org

Change-Id: I8c617515b34eb2d262d58a239a29c1515de2d92d
Reviewed-on: https://chromium-review.googlesource.com/c/1417611
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58892}