[llvm] [IVDescriptors] Implement MonotonicDescriptor (PR #214490)

[Gaëtan Bossu via llvm-commits]

https://github.com/gbossu approved this pull request.

Thanks for the changes. Now that `MonotonicDescriptor` does not try to incorporate LV legality checks anymore and matches a plain "conditional IV", this LGTM :)

https://github.com/llvm/llvm-project/pull/214490