[llvm] [NFC][MachineLICM] Use SmallDenseSet instead of SmallSet (PR #95201)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

https://llvm-compile-time-tracker.com/compare.php?from=1b66306c9c1adce20e2f3cfb1df0af2fb6a10318&to=432ec88ec392e00782e2e165e9175581ed4771d3&stat=instructions%3Au

https://github.com/llvm/llvm-project/pull/95201