[llvm] [FlattenCFG] Fix an Imprecise Usage of AA (PR #128117)

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Chengjunp wrote:

Hi @nikic, thank you so much for your review. Could you help to merge the change? 

I'm also working on https://github.com/llvm/llvm-project/pull/125965. It would be great if you can also help to review that PR.

https://github.com/llvm/llvm-project/pull/128117