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

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mrdaybird wrote:

Anyway, I have fixed the test error. Also, I added the check that Stride(LHS) should be Positive, I mistakenly assumed it was implied. 
For the current code, (left_stride > 0 and right_stride < 0), Alive2 proof(with unrolling): https://alive2.llvm.org/ce/z/f2CpXv

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