LiveRegUnits: Port recent LivePhysRegs bugfixes

Adjust code to look more like the code in LivePhysRegs and port over the
fix for LivePhysRegs from r304001 and adapt to the new CSR management in
MachineRegisterInfo.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304622 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed