[VPlan] Add abstract base class for header phi recipes (NFC).

Not all header phis widen the phi, e.g. like the new
VPCanonicalIVPHIRecipe in D113223. To let those recipes also inherit
from a phi-like base class, add a more generic VPHeaderPHIRecipe
abstract base class.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D116304