[AArch64] Optimize CBZ wzr and friends. (#161508)
In certain situations, especially with zero phi operands propagated after tail
duplications, we can end up with CBZ/CBNZ/TBZ/TBNZ with a zero register. It
can can be introduced late in the pipeline.
This patch adds a basic pass to fold them away to either a direct branch or
removing the instruction entirely. It runs quite late n the pipeline, so doesnt
fit into any of the existing passes. It only needs to look at the terminators
to each BB, so the new pass should have a limited in compile-time impact.
