ESSL 1.00 spec section 5.9 says that equality operators don't operate on
structs containing sampler types. Section 5.7 also suggests this. ESSL
3.00 doesn't have a similar restriction.

ESSL 1.00 spec section 4.1.7 says that structs containing samplers can't
be used as l-values. This is interpreted to apply also in the case of
ESSL 3.00, which similarly disallows samplers as l-values, but doesn't
explicitly mention structs.

BUG=angleproject:954
TEST=angle_unittests, WebGL conformance tests

Change-Id: I73f74962a192e8d9449990ffa5f3d8c851491601
Reviewed-on: https://chromium-review.googlesource.com/261822Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>