[llvm] [AArch64] Extend usage of `XAR` instruction for fixed-length operations (PR #139460)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

I think it will need a `INSERT_SUBREG IMPLICIT_DEF, A, zsub` for the input and a `EXTRACT_SUBREG xar, zsub` to make sure the result is kept as the right type for the result.

It can apply to both the rotr(xor(a, b))->xar(a,b) and the rotr(a)->xar(a,0) versions (so it might be easier to expand R1 and R2.

https://github.com/llvm/llvm-project/pull/139460