[VPlan] Allow building a VPlan to may fail.

Update the planning code constructing VPlan to allow building VPlans to
fail. This allows us to gradually shift some legality checks to VPlan
construction. The first candidate is checking if all users of
first-order recurrence phis can be sunk past the recipe computing the
previous value.

The new functionality will be used by D142886 which is approved and will
be landed shortly.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D142885