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

[Sergei Barannikov via llvm-commits]

https://github.com/s-barannikov commented:

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

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