[VPlan] Print live-in backedge taken count as part of plan.

At the moment, a VPValue is created for the backedge-taken count, which
is used by some recipes. To make it easier to identify the operands of
recipes using the backedge-taken count, print it at the beginning of the
VPlan if it is used.

Reviewed By: a.elovikov

Differential Revision: https://reviews.llvm.org/D111298