[llvm] [SimplifyCFG] Avoid increasing too many phi entries when removing empty blocks (PR #104887)

[via llvm-commits]

Chengjunp wrote:

Hi @nikic, I took any look at the code, and I think it could be a little bit complex to track the total number of new phi entries of different blocks in a pass since the function `TryToSimplifyUncondBranchFromEmptyBlock` could be used in different passes. Could you help to review the current change and do a compile time test on it? Thank you very much!

https://github.com/llvm/llvm-project/pull/104887