[llvm] [NFC] Use more efficient data structures in SubtargetEmitter (PR #102090)

[Rahul Joshi via llvm-commits]

jurahul wrote:

> They might be there for a reason. Please make sure unordered containers don't add introduce nondeterministic behavior.

Yeah, I did check that these containers are only used for membership check and not iterated over, so the change should not cause nondeterminism.

https://github.com/llvm/llvm-project/pull/102090