[VPlan] Merge main VPlan verifer with HCFG verifier.

Unify VPlan verifiers in verifyVPlanIsValid. This adds verification for
various properties on blocks to the verifier used for VPlans generated
by the inner loop vectorizer. It also adds def-use checks for the
verifier used in the VPlan native path.

This drops the separate flag to enable HCFG verification. Instead, all
VPlans are verified once they have been created, if assertions are
enabled.

This also removes VPWidenPHIRecipe from VPHeaderPHIRecipe; it is used to
model any phi node in the native path.