RAGreedy: Keep track of allocated PhysRegs internally

Do not use MachineRegisterInfo::setPhysRegUsed()/isPhysRegUsed()
anymore. This bitset changes function-global state and is set by the
VirtRegRewriter anyway.
Simply use a bitvector private to RAGreedy.

Differential Revision: http://reviews.llvm.org/D10910

llvm-svn: 242169