Directly compute the cost of a VPlan after construction and track it
together with a plan. This allows moving selecting the best VF to the
planner. This seems to be a good fit anyways, and removes code from the
cost-model that is not directly related to assigning costs to a specific
plan/VF. Later this can be swapped out with computing the cost for a
plan directly.

This may help to simplify D142015.

Differential Revision: https://reviews.llvm.org/D143938

This patch follows D89322 to add an initial skeleton of vplan-based cost model.

This difference is that instead of incorporating a cost() interface to VPRecipes,
all cost implementations are put together in VPlanCostModel.

This allows VPlanCostModel to concentrate on assigning costs to vplan,
thus seprating the cost model code from the vplan IR, similar to LLVM IR cost
modeling.

During the transition, it will still use the legacy model to obtain cost until
all cost calculation for recipes are implemented.

Please let me know if you agree with the main idea of this patch.
If there is a general consensus, I'll proceed to implement the cost for the
other recipes for review.

Differential Revision: https://reviews.llvm.org/D158716

- Address comments
- Move VPCM object outside of the loop
- Add getElementType() and getReturnElementType()