Move isSubRegOf into MRegisterInfo. Fix a missed move elimination in LowerSubregs and add more debugging output there.

llvm-svn: 41005

llvm-svn: 40863

Add a MachineFunction pass, which runs post register allocation, that turns subreg insert/extract instruction into register copies. This ensures correct code gen if the coalescer isn't able to remove all subreg instructions.

llvm-svn: 40521