[llvm] [MachineSink] Check predecessor/successor relationship between two basic blocks involved in critical edge splitting (PR #98540)

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yozhu wrote:

> I'd like to get us unblocked before the weekend. I hope you won't mind me merging this PR.

Oh, not at all; and thanks for merging this PR :)

https://github.com/llvm/llvm-project/pull/98540