Add an MF argument to MachineInstr::addOperand().

Just like for addMemOperand(), the function pointer provides a context
for allocating memory. This will make it possible to use a better memory
allocation strategy for the MI operand list, which is currently a slow
std::vector.

Most calls to addOperand() come from MachineInstrBuilder, so give that
class an MF reference as well. Code using BuildMI() won't need changing
at all since the MF reference is already required to allocate a
MachineInstr.

Future patches will fix code that calls MI::addOperand(Op) directly, as
well as code that uses the now deprecated MachineInstrBuilder(MI)
constructor.

llvm-svn: 170574