Ensure that for setcc, we only use a byte register as destination
register.

R=titzer@chromium.org

Bug: v8:6600, chromium:800756
Change-Id: Ie33f3faf602e7eda845205ba0ed2d9966460fd54
Reviewed-on: https://chromium-review.googlesource.com/860640Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50508}