[llvm] [AArch64][SME] Add missing SMStartStop regmasks (PR #68458)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm requested changes to this pull request.

In contrast to `smstart sm`, the instructions `smstart za` and `smstop za` don't clobber any registers as they merely just enable access to ZA. It's strictly the change to/from streaming-SVE mode that clobbers any AdvSIMD/FP/SVE state. So this change is not necessary.

https://github.com/llvm/llvm-project/pull/68458