[llvm] [Transforms/Utils][NFC] Replace SmallPtrSet with vector (PR #186664)

[Alexis Engelke via llvm-commits]

aengelke wrote:

[stage2-O3 -0.11%](http://llvm-compile-time-tracker.com/compare.php?from=a7aebd809d49fd1c8205e8f0fcdc8d97777e5103&to=7dbbc23bdddf54d9e18dd997538bea64b06ed4ac&stat=instructions:u)

https://github.com/llvm/llvm-project/pull/186664