CodeGen: Add/Factor out LiveRegUnits class; NFCI
This is a set of register units intended to track register liveness, it
is similar in spirit to LivePhysRegs.
You can also think of this as the liveness tracking parts of the
RegisterScavenger factored out into an own class.
This was proposed in http://llvm.org/PR27609
Differential Revision: http://reviews.llvm.org/D21916