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

[via llvm-commits]

hiraditya wrote:

> Then number of iterations will be (right_start - left_start)/(s1-(-s2)).

What is the number of iterations when s1+s2 can't exactly divide (right_start - left_start)? It might be a `(right_start - left_start)/(s1-(-s2)) + 1`

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