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

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Chengjunp wrote:

> Does this patch still cover your original motivating case, or is the new version not sufficient due to this problem?

Sorry for the late reply. I have done the tests during the last few days and the current version can help improve the compile time in my original case. If everything looks fine, could you help review and merge this change? Thanks!

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