Fix unused variable warning when compiling with asserts disabled.

[bkramer]

The code now essentially says

if (OldMO.isDef())
  assert(OldMO.isDef())

Which seems redundant. But I guess getting rid of the warning is fine.

[jmorse]

Hrrmmm, that smells fishy; thanks for the warning sqelching, I'll follow it up.

llvm/lib/CodeGen/MachineFunction.cpp

if (!OldInstrNum)
     return;
 
   // Iterate over all operands looking for defs to create substitutions for.
   // Avoid creating new instr numbers unless we create a new substitution.
   // While this has no functional effect, it risks confusing someone reading
   // MIR output.
   for (unsigned int I = 0; I < Old.getNumOperands(); ++I) {
     const auto &OldMO = Old.getOperand(I);
-    auto &NewMO = Old.getOperand(I);
 
     if (!OldMO.isReg() || !OldMO.isDef())
       continue;
-    assert(NewMO.isDef());
+    assert(Old.getOperand(I).isDef());
 
     unsigned NewInstrNum = New.getDebugInstrNum();
     makeDebugValueSubstitution(std::make_pair(OldInstrNum, I),
                                std::make_pair(NewInstrNum, I));
   }
 }
 
 /// \}