[VPlan] Model first exit values using VPLiveOut.

This patch introduces a new VPLiveOut subclass of VPUser  to model
 exit values explicitly. The initial version handles exit values that
are neither part of induction or reduction chains nor first order
recurrence phis.

Fixes #51366, #54867, #55167, #55459

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D123537