Fix initialization of const references

Reference lifetime extension only applies in this case

  const T& R = T(...);

or

  const T& R = f(...);  // where f returns T

This means the temporary bound to R will be destroyed at the end of the
block so R can be safely used until then.

However it does not apply in this case

  const T& R = T(...).modifier(...);

not even if T::modifier returns 'const T&'. It doesn't matter: the
temporary T() will be destroyed at the ';"

MachineInstrBuilder has a particularly annoying error-prone API that
leads to these kind of errors.

Let's not use 'const MachineInstrBuilder &' ever. Not even when it could
work: just in case later we decide to extend the instruction being built
calling a 'modifier'.