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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

The general approach here seems fine; there are some special-cases you could optimize, but we can leave that for a followup.

Please refactor the copy-pasted code so the relevant codepaths are shared.

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