glsl/ast: Generalize some sampler variable restrictions to all opaque types.

glsl/ast: Generalize some sampler variable restrictions to all opaque types.

No opaque types may be statically initialized in the shader, all
opaque variables must be declared uniform or be part of an "in"
function parameter declaration, no opaque types may be used as the
return type of a function.

v2: Add explicit check for opaque types in interface blocks.  Check
    for opaque types in ir_dereference::is_lvalue().

Reviewed-by: Paul Berry <stereotype441@gmail.com>