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

[via llvm-commits]

mrdaybird wrote:

Alive2 proof: https://alive2.llvm.org/ce/z/pFJeUx -> checks for validity for all start values as well as all strides(i.e. s1, s2 such that s1>0 and s2<0 and (s1-s2) does not overflow)

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