[VPlan] Handle IV vector splat using VPWidenCanonicalIV.

This patch tries to use an existing VPWidenCanonicalIVRecipe
instead of creating another step-vector for canonical
induction recipes in widenIntOrFpInduction.

This has the following benefits:

 1. First step to avoid setting both vector and scalar values for the
    same induction def.
 2. Reducing complexity of widenIntOrFpInduction through making things
    more explicit in VPlan
 3. Only need to splat the vector IV for block in masks.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D116123