No test case covering sampler operands

ESSL 1.00 section 4.1.7, and ESSL 3.00 section.
Except for parameters to texture lookup functions, array indexing, structure field selection, and parentheses, samplers are not allowed to be operands in expressions.

Better to have some test cases to cover the rule. An ANGLE bug(https://bugs.chromium.org/p/angleproject/issues/detail?id=2028) can then be exposed.

Start by locating the WebGL conformance tests for ESSL expression validation and the existing sampler-related cases. Add coverage for the operand restrictions described in ESSL 1.00 section 4.1.7 and ESSL 3.00, including the allowed contexts and disallowed expression operands; done means the tests expose violations such as the referenced ANGLE bug.

Written by the indexing model from the issue text.