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

[Rahul Joshi via llvm-commits]

jurahul wrote:

@tmatheson-arm I lost my commit access in the purge that happened earlier this year. Would you be able to merge this PR as well? Thanks!

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