Right-shifting the minimum signed integer needs to be handled as a
special case, since it can't go through the usual path that clears the
sign bit.

Code for right-shifting by zero also had a typo that resulted in
setting the wrong value to the result.

BUG=chromium:662706
TEST=angle_unittests

Change-Id: Ief24d738064906a72212242e0917ce30e45d6b25
Reviewed-on: https://chromium-review.googlesource.com/408158Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>