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

[via llvm-commits]

mrdaybird wrote:

@efriedma-quic From what I understand, if we could not find the exact backedge-count(like for the cases you mentioned) we return SCEVCouldNotCompute, which is considered greater than any computable value. 

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