[VPlan] Add InductionDescriptor to VPWidenIntOrFpInduction. (NFC)

This allows easier access to the induction descriptor from VPlan,
without needing to go through Legal. VPReductionPHIRecipe already
contains a RecurrenceDescriptor in a similar fashion.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D115111