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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

The case I was thinking of is something like https://alive2.llvm.org/ce/z/kzxOYg .  Alive2 thinks it's fine as long as we keep the strides-not-equal check.

And yes, we can always return SCEVCouldNotCompute() if we can't prove there's a safe value to return.

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