[llvm] [SCEV] Handle backedge-count logic for std::reverse like loops (PR #92560)

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mrdaybird wrote:

@efriedma-quic I am not sure what exactly needs to be done. The case that you mentioned is true in general(i.e. for any type of loop), therefore shouldn't it be already handled by the previous code. Maybe I am misunderstanding you point here? If so, maybe if you could refer me to the original alive2 proof for computing backedge count (or something similar) it would point me in the right direction!

https://github.com/llvm/llvm-project/pull/92560