Mask the lower byte before doing the compare.

Bug: v8:5717, v8:6455
Change-Id: I0c7e8b79adc36fb5ee643eae2e42fd892cd560fd
Reviewed-on: https://chromium-review.googlesource.com/527885
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46070}