[IVDescriptors] Add pointer InductionDescriptors with non-constant strides

This matches the handling for integer IVs.  I left the non-opaque cases alone, mostly because they're largely irrelevant today.

This doesn't actually make much difference in vectorization right now as we immediately fail on aliasing checks (which also bail on non-constant strides).  Slightly suprisingly, it's the case which *do* need runtime checks which work after this patch as they don't use the same dependency analysis path.

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