[IVDescriptors] Add pointer InductionDescriptors with non-constant strides (try 2)

(JFYI - This has been heavily reframed since original attempt at landing.)

This change updates the InductionDescriptor logic to allow matching a pointer IV with a non-constant stride, but also updates the LoopVectorizer to bailout on such descriptors by default. This preserves the default vectorizer behavior.

In review, it was pointed out that there's multiple unfortunate performance implications which need to be addressed before this can be enabled. Having a flag allows us to exercise the behavior, and write test cases for logic which is otherwise unreachable (or hard to reach).

This will also enable non-constant stride pointer recurrences for other consumers. I've audited said code, and don't see any obvious issues.

Differential Revision: https://reviews.llvm.org/D147336