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

[Scott Linder via llvm-commits]

slinder1 wrote:

> 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).

Thank you for the pointer! Nikita graciously added my fork and I tried it out on this branch. The worst benchmark for instructions:u is `+0.03%` and it all seems well within the noise.

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