panfrost/midgard: Implement "pipeline register" prepass

panfrost/midgard: Implement "pipeline register" prepass

This prepass, run after scheduling but before RA, specializes to
pipeline registers where possible. It walks the IR, checking whether
sources are ever used outside of the immediate bundle in which they are
written. If they are not, they are rewritten to a pipeline register (r24
or r25), valid only within the bundle itself. This has theoretical
benefits for power consumption and register pressure (and performance by
extension). While this is tested to work, it's not clear how much of a
win it really is, especially without an out-of-order scheduler (yet!).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Ryan Houdek <Sonicadvance1@gmail.com>