This adds a new computeCost interface to VPReicpeBase and implements it
for VPWidenRecipe and VPWidenIntOrFpInductionRecipe.

It also adds getBestPlan function to LVP which computes the cost of all
VPlans and picks the most profitable one together with the most
profitable VF. For recipes that do not yet implement computeCost, the
legacy cost for the underlying instruction is used.

The VPlan selected by the VPlan cost model is executed and there is an
assert to catch cases where the VPlan cost model and the legacy cost
model disagree.