[llvm] [JumpThreading] Use [BB->SuccIndx] to get probability when updating BB info. (PR #134585)

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tianleliu wrote:

Hi Nikic, thanks for your review. I have added branch weight checking in test file.
I think dumping branch probability is helpful to understand what the patch changes. So I prefer to remain it?

https://github.com/llvm/llvm-project/pull/134585