Quoting the ESSL 1.00 spec section 4.1.7: "Except for parameters to
texture lookup functions, array indexing, structure field selection,
and parentheses, samplers are not allowed to be operands in
expressions."

ESSL 3.00 has a similar passage related to opaque types.

Validate this correctly. Compatibility risk should be low, since
attempting to use samplers in ternary operators was already failing
before this in most cases.

BUG=angleproject:1551
TEST=angle_unittests

Change-Id: I6cbb536f473ba9674d558b14a458f3799f9c7c9c
Reviewed-on: https://chromium-review.googlesource.com/402694Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>