[VPlan] Add prepareToExecute to set up live-ins (NFC).

This patch adds a new prepareToExecute helper to set up live-ins, so
VPTransformState doesn't need to hold values like TripCount.

This also requires making the trip count operand for ActiveLaneMask
explicit in VPlan.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D116320