[VPlan] Remove dead header-phi recipes.

[VPlan] Remove dead header-phi recipes.

This patch adds a new transform to remove dead recipes. For now, it only
removes dead recipes in the header, to keep the number tests that require
updating manageable. Future patches will extend this to remove dead
recipes across the whole plan.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D118051