[llvm] [CodeGen] Use SmallSetVector for getRegAllocationHints (PR #224472)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Can we run this through the compiler time tracker? `SmallSetVector` is presumably more expensive than `SmallVector`, and it would be good to ensure that there are no compile time regressions from this as I believe some of these code paths are relatively hot.

Otherwise, the patch generally LGTM and it seems like the right approach.

https://github.com/llvm/llvm-project/pull/224472