MachineVerifier::visitMachineFunctionAfter() is extended to check the
live-through case for live-in lists. This is only done for registers without
aliases and that are neither allocatable or reserved, such as the SystemZ::CC
register.
The MachineVerifier earlier only catched the case of a live-in use without an
entry in the live-in list (as "using an undefined physical register").
A comment in LivePhysRegs.h has been added stating a guarantee that
addLiveOuts() can be trusted for a full register both before and after
register allocation.
Review: Quentin Colombet
Differential Revision: https://reviews.llvm.org/D68267
