nir: generalize bitfield_reverse bit size

No reason we can't reverse other bit sizes, we just need to generalize the
constant folding & bit size lowering.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35198>
4 files changed