[llvm] [FlattenCFG] Fixup Phi nodes during CFG flattening (PR #143766)

[via llvm-commits]

HighW4y2H3ll wrote:

@nikic I fixed up the FlattenCFG pass which shall be able to handle this CFG now! Could you take another look when its convenient? Thank you! :) 

https://github.com/llvm/llvm-project/pull/143766