ANGLE recently gained the ability to constant fold some expressions that
are not constant expressions. ValidateLimitations should continue to
recognize all cases where an expression is not a constant expression.

BUG=angleproject:851
TEST=angle_unittests

Change-Id: I8ad0552a59213cdd6af9a220ffd672be9752271d
Reviewed-on: https://chromium-review.googlesource.com/317281Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>