[VPlan] Support sinking VPScalarIVStepsRecipe.

This patch extends VP-based sinking to also sink VPScalarStepsRecipe.
This takes us a step closer towards retiring the IR based sinking.

The main change is extending VPScalarIVStepsRecipe::execute to support
executing in a replicate-region.

Depends on D133758.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D133760