[CodeGen] Use MachineOperand::print in the MIRPrinter for MO_Register.
Work towards the unification of MIR and debug output by refactoring the interfaces.
For MachineOperand::print, keep a simple version that can be easily called from dump(), and a more complex one which will be called from both the MIRPrinter and MachineInstr::print.
Add extra checks inside MachineOperand for detached operands (operands with getParent() == nullptr).
too much automatic replacements :) (and too fancy writing in the original comment)