Fix ESSL3 indexing corner cases 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/312044 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org>