Convert Alpha and Mips to use a MachineFunctionInfo subclass to

carry GlobalBaseReg, and GlobalRetAddr too in Alpha's case. This
eliminates the need for them to search through the
MachineRegisterInfo livein list in order to identify these
virtual registers. EmitLiveInCopies is now the only user of the
virtual register portion of MachineRegisterInfo's livein data.

llvm-svn: 72802