glsl: Verify that open-code bitfieldReverse is not improperly optimized

I observed that on Sandybridge several Unreal Engine 4 tech demos tried to generate BFREV instrucitons that are not supported on that platform.

This test doesn't seem like it belongs in tests/spec/..., but I did a version for GLSL 1.30 and one of GL_EXT_gpu_shader4 because I plan to support the latter on a couple platforms that cannot do GLSL 1.30. How do people feel about the naming used for the tests in this case?