- In thumb mode, if size of MachineFunction is >= 2048, force LR to be

  spilled (if it is not already).
- If LR is spilled, use BL to implement far jumps. LR is not used as a GPR
  in thumb mode so it can be clobbered if it is properly spilled / restored
  in prologue / epilogue.
- If LR is force spilled but no far jump has been emitted, try undo'ing the
  spill by:
  push lr -> delete
  pop pc -> bx lr

llvm-svn: 33650