glsl: Add type predicate to check whether a type contains any opaque types.

glsl: Add type predicate to check whether a type contains any opaque types.

And use it to forbid comparisons of opaque operands.  According to the
GL 4.2 specification:

> Except for array indexing, structure member selection, and
> parentheses, opaque variables are not allowed to be operands in
> expressions.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>