[llvm] [NFC][AsmPrinter] Refactor FrameIndexExprs as a std::set (PR #66433)

[Stephen Tozer via llvm-commits]

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

Definitely looks nicer with std::set than the mutable; if compile times are a concern, you can also try using the [LLVM compile time tracker](https://llvm-compile-time-tracker.com/) - you could get set up to use it to test commits pre-merge, but you can also just see if your patch causes any large changes post-merge (which is fine imo given the numbers you posted on the earlier patch).

https://github.com/llvm/llvm-project/pull/66433