[VPlan] Don't add live-outs if scalar epilogue is required.

Instead of clearing live outs when a scalar epilogue is required late,
don't add live outs during VPlan construction if a scalar epilogue is
required.

This enables more VPlan-based DCE (if the live out would be the only
user in the plan) and is a step towards removing an access of the cost
model in fixedVectorizedLoop (which is after VPlan execution).

Depends on D147468.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D147471