[llvm] [AArch64][SME] Enable subreg liveness tracking when SME is available (PR #92142)

[David Green via llvm-commits]

davemgreen wrote:

This sounds like a nice idea - I hadn't realized we did not have this enabled already. Whilst I can see how it would be especially beneficial for SME's multi-vector instructions, I feel it should either be useful in general (for tbl's and ld2/3/4 type instructions), or be wrong as something doesn't handle the subregister liveness correctly. In either case, what do you think about enabling this in general for AArch64 so we can be sure it performs correctly?

https://github.com/llvm/llvm-project/pull/92142