[llvm] [AArch64] Refactor PACM emission in Pointer Authentication (NFC) (PR #175937)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

LGTM 

I agree the code is easier to follow as the BuildPACM may do nothing, which isn't obvious from the name, and some of the if conditions are redundant as they are done at the call site.

We do lose the comment about PACM being in the hint space, but I think that can be easily looked up if needed, rather than repeating it 3 times.  

https://github.com/llvm/llvm-project/pull/175937