[llvm] [LoopBoundSplit] Fix edge connections during transformation (PR #192106)

[Congzhe Cao via llvm-commits]

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CongzheUalberta wrote:

I'm not sure if I get your comment entirely correctly, could you please clarify a bit?

I think this doesn't update DomTree because `PostLoop->getHeader()` already has `PostLoopPreHeader` as its immediate dominator, and `PostLoop->getExitBlock()` does not have `PostLoopPreHeader` to be its immediate dominator so no need to update. 

https://github.com/llvm/llvm-project/pull/192106