Indexing interface blocks or fragment outputs with a non-constant
expression is not valid even if ANGLE has been able to constant fold
the expression.

BUG=angleproject:1210
TEST=angle_unittests

Change-Id: I2ccb67871b682976a31b8de306053b9b28c06437
Reviewed-on: https://chromium-review.googlesource.com/312044Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>