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

[Florian Hahn via llvm-commits]

fhahn wrote:

> > The current name may be confusing in some cases, as IIUC we do not require the described chain to actually be monotonic (i.e. check that it does not wrap)?
> 
> Would you like me to rename things before proceeding with #214491? The initial matching does not require no-wrap flags, but that prevents vectorization. Ultimately, we should add a SCEV predicate (similar to the standard `InductionDescriptor`), if no-wrap is not specified.

I think we should either rename it if we want valid `MonotonicDescriptor` to not necessarily monotonic or require them to be monotonic. The latter would probably be preferable. If SCEV predicates are needed to make it monotonic, they should be added during the initial analysis, like for InductionDescriptor, unless that's not possible for some reason.

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