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

[Florian Hahn via llvm-commits]

fhahn wrote:

> > 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.
> 
> That's what I was planning, though I was considering it as a follow-up once the basic support for compressing patterns lands.

I'm not suggesting this part should be done now, just fixing `isMonotonicPHI` so it only returns true for monotonic phis (checking flags) or rename it, although the former would be preferable unless there's a reason to allow such cases and require the only consumer to reject it later

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