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

[via llvm-commits]

mrdaybird 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`

In the code, backedge count is calculated as ceil((End - Start) /u (Stride - RHSStride) where, End = max(RHSStart, Start). 


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