Use MachineInstrBuilder in InstrEmitter.

This is supposed to be a mechanical change with no functional effects.

InstrEmitter can generate all types of MachineOperands which revealed
that MachineInstrBuilder was missing a few methods, added by this patch.

Besides providing a context pointer to MI::addOperand(),
MachineInstrBuilder seems like a better fit for this code.

llvm-svn: 170712