[llvm] [AArch64][SME] Propagate desired ZA states in the MachineSMEABIPass (PR #149510)

[Benjamin Maxwell via llvm-commits]

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MacDue wrote:

`ZAState::OFF` can't be added here, as the requirement for a bundle state is that it's possible to transition to/from that state without losing the contents of ZA. `ZAState::OFF` does not meet that requirement as it drops the contents of ZA.

If we were to optimize this case, I think the correct check would be to exit early after `collectNeededZAStates()` if no instructions needing ZA were found. 

https://github.com/llvm/llvm-project/pull/149510