Add a MachineFunction pass, which runs post register allocation, that turns subreg...

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@40521 91177308-0d34-0410-b5e6-96231b3b80d8