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

[Nikita Popov via llvm-commits]

nikic wrote:

@huangjd The files are from https://github.com/dtcxzyw/llvm-opt-benchmark. E.g. this is the one with the largest regression: https://github.com/dtcxzyw/llvm-opt-benchmark/blob/main/bench/lightgbm/original/application.cpp.ll It should be reproducible by running it through `opt -O3`.

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