NIR: Don't optimize open-code bitfieldReverse when bitfieldReverse lowering is enabled

Found while working on something else. I didn't actually run on Sandybridge, but I was able to confirm using INTEL_DEVID_OVERRIDE that without this series a BFREV instruction is incorrectly generated. With this series, BFREV is not generated on Sandybridge.

See also piglit!122 (merged).

This may mean that some of the GLSL IR lowering could be removed, but I haven't poked at that yet. That may be a follow-up series.