[VPlan] Add VPWidenPointerInductionRecipe.

This patch moves pointer induction handling from VPWidenPHIRecipe to its
own recipe. In the process, it adds all information required to generate
code for pointer inductions without relying on Legal to access the list
of induction phis.

Alternatively VPWidenPHIRecipe could also take an optional pointer to InductionDescriptor.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D121615