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

[fhahn]

Summary

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

Split off from D119661.

[Ayal]

Thanks for addressing, adding a couple of minor suggestions.

llvm/lib/Transforms/Vectorize/VPlan.h

1137

Phi-like recipes are kept in order to facilitate checking intervals (and help avoid such missed-case bugs)?

llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp

136–139

Perhaps worth outlining the first part that verifies phi recipes, breaking the somewhat long method into two?

[fhahn]

llvm/lib/Transforms/Vectorize/VPlan.h

1137

I'll do that separately.

llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp

136–139

Done!