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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

@boomanaiden154 numbers LGTM: https://llvm-compile-time-tracker.com/compare.php?from=cdbca60c7d5f41ec7f332341f97f16e4e2e221cb&to=58df225acc35c256448fcfc8b183305363f99611&stat=instructions:u

thanks @nikic !

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