[VPlan] Change recipes to inherit from VPUser instead of a member var.

Now that VPUser is not inheriting from VPValue, we can take the next
step and turn the recipes that already manage their operands via VPUser
into VPUsers directly. This is another small step towards traversing
def-use chains in VPlan.

This is NFC with respect to the generated code, but makes the interface
more powerful.