This flag generates false positives, since gcc inlines functions and
propagates constants, and then applies the check.

Drive-by: Refactor the checks that triggered the error to avoid
explicit casts.

R=jochen@chromium.org, machenbach@chromium.org
BUG=v8:6341

Change-Id: I86aebf402cbd2502ef17622a000a5bb777fd4b43
Reviewed-on: https://chromium-review.googlesource.com/494474Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45096}