Minor simplifications and an additional overflow check.

R=mstarzinger@chromium.org

Bug: v8:8238
Change-Id: I169464319a0e70562f3a443f429e462d30dd2fa3
Reviewed-on: https://chromium-review.googlesource.com/c/1296482Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56929}