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

[William Junda Huang via llvm-commits]

huangjd wrote:

Flame graph comparison

before

![Screenshot from 2024-07-23 19-46-51](https://github.com/user-attachments/assets/9a74d2a0-ef3a-4381-9661-baa3a71d0406)

after

![Screenshot from 2024-07-23 19-47-19](https://github.com/user-attachments/assets/40536444-b8fa-4638-b791-06792bf69111)


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