[LV] Apply sink-after & interleave-groups as VPlan transformations (NFC)

The sink-after and interleave-group vectorization decisions were so far applied to
VPlan during initial VPlan construction, which complicates VPlan construction – also because of
their inter-dependence. This patch refactors buildVPlanWithRecipes() to construct a simpler
initial VPlan and later apply both these vectorization decisions, in order, as VPlan-to-VPlan
transformations.

Differential Revision: https://reviews.llvm.org/D68577