Remove the explicit MachineInstrBuilder(MI) constructor.

Use the version that also takes an MF reference instead.

It would technically be possible to extract an MF reference from the MI
as MI->getParent()->getParent(), but that would not work for MIs that
are not inserted into any basic block.

Given the reasonably small number of places this constructor was used at
all, I preferred the compile time check to a run time assertion.

llvm-svn: 170588