[VPlan] Use VPLiveOut to update FOR live-out users.

Instead of iterating over all LCSSA phis in the exit block, collect all
LiveOut users of the FOR splice VPInstruction and only update those
users.

Building on top of D147471, this removes an access to the cost model
after VPlan execution.

Depends on D147471.

Reviewed By: Ayal, michaelmaitland

Differential Revision: https://reviews.llvm.org/D147472