- Register scavenger should use MachineRegisterInfo and internal map to find...

- Register scavenger should use MachineRegisterInfo and internal map to find the first use of a register after a given machine instruction.
- When scavenging a register, in addition to the spill, insert a restore before the first use.
- Abort if client is looking to scavenge a register even when a previously scavenged register is still live.

llvm-svn: 59697