[llvm] [AArch64] Lower cttz(bitcast <Nxi1> to iN) with shrn-based compressed movemask (PR #199081)

[Adam Scott via llvm-commits]

as4230 wrote:

Thanks so much for the reviews and the patience on this one @efriedma-quic . I don't have commit access, so would you be able to land it for me?

https://github.com/llvm/llvm-project/pull/199081