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

[Sander de Smalen via llvm-commits]

sdesmalen-arm 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?

I suspect this may not be enabled by default for compile-time reasons. It's worth doing some measurements on what the impact is and decide based on that whether we want to enable it for more cases.

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