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

[Benjamin Maxwell via llvm-commits]

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

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