[VPlan] use getVPValueOrAddLiveIn in VPlan::duplicate.

Instead of creating live-ins manually, use getOrAddLiveIn which
automatically takes care of adding them to VPLiveInsToFree. Also use it
to create the VPValue for the trip-count. This fixes a leak:
https://lab.llvm.org/buildbot/#/builders/168/builds/18308/steps/10/logs/stdio