[LV][VPlan] Fix printing TripCount liveins. NFC

The TripCount liveins would currently be printed as badref in the vplan as they
are not allocated slots in the VPSlotTracker. This patch allocates them a slot
and adds them to the printed Live-Ins. It also makes a minor adjustment to
printing of Live-ins to reduce the empty lines when multiple Live-ins are
present.

Differential Revision: https://reviews.llvm.org/D145507