Phase 2 of the great MachineRegisterInfo cleanup.  This time, we're changing
operator* on the by-operand iterators to return a MachineOperand& rather than
a MachineInstr&.  At this point they almost behave like normal iterators!

Again, this requires making some existing loops more verbose, but should pave
the way for the big range-based for-loop cleanups in the future.