[llvm] [AArch64] Optimize CBZ wzr and friends. (PR #161508)

[David Tellenbach via llvm-commits]

dtellenbach wrote:

Would it make sense to also check for `cb.{eq,ne} {x,w}zr, #0`? Not sure if we'd ever codegen that though.

https://github.com/llvm/llvm-project/pull/161508