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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Thinking about it a bit more, there's one thing the alive2 proof currently isn't testing: other exits from the loop (or infinite loops nested inside the loop).  In those cases, even if we prove it's impossible to exit via this exit, we need to ensure that the returned backedge-taken count is greater than the number of times the loop actually iterates.

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