[llvm] [FlattenCFG] Fix the miscompilation where phi nodes exist in the merge point (PR #81987)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Can the code that was added by [d98cb81](https://github.com/llvm/llvm-project/commit/d98cb81cd11c8a4d7017c4820cae202d31d757bb) be dropped now?

Yeah, the original patch(https://reviews.llvm.org/D68032) just fixed the crash in an incorrect way.

https://github.com/llvm/llvm-project/pull/81987