This saves a few control merges in Liftoff, and might also generate
smaller graphs in Turbofan.

R=titzer@chromium.org

Bug: v8:6600, v8:8423
Change-Id: Ice921f8b048809bc38b820b94688f482e67bd386
Reviewed-on: https://chromium-review.googlesource.com/c/1354039Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57907}