[llvm] [Transforms] Speed up SSAUpdater::FindExistingPHI (PR #100281)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

@huangjd Can you please analyze the cases with compile-time regressions that @dtcxzyw mentioned? The patch itself looks fine to me.

https://github.com/llvm/llvm-project/pull/100281