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

[via llvm-commits]

mrdaybird wrote:

@efriedma-quic Updated alive2 proof: https://alive2.llvm.org/ce/z/sXwLZY. 
Also, fixed the comments.

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