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

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

Can the code that was added by d98cb81cd11c8a4d7017c4820cae202d31d757bb be dropped now?

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