[VPlan] Verify that header only contains header phi recipes.

Add verification that VPHeaderPHIRecipes are only in header VPBBs. Also
adds missing checks for VPPointerInductionRecipe to
VPHeaderPHIRecipe::classof.

Split off from D119661.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D131989