[MISched] Explanatory error message when machine model is not complete. NFC

[MatzeB]

LGTM

lib/CodeGen/TargetSchedule.cpp

unsigned TargetSchedModel::computeOperandLatency(
   }
   // If DefIdx does not exist in the model (e.g. implicit defs), then return
   // unit latency (defaultDefLatency may be too conservative).
 #ifndef NDEBUG
   if (SCDesc->isValid() && !DefMI->getOperand(DefOperIdx).isImplicit()
       && !DefMI->getDesc().OpInfo[DefOperIdx].isOptionalDef()
       && SchedModel.isComplete()) {
     errs() << "DefIdx " << DefIdx << " exceeds machine model writes for "
-           << *DefMI;
+           << *DefMI << " (Try with MCSchedModel.CompleteModel set to false)";
     llvm_unreachable("incomplete machine model");
   }
 #endif
   // FIXME: Automatically giving all implicit defs defaultDefLatency is
   // undesirable. We should only do it for defs that are known to the MC
   // desc like flags. Truly implicit defs should get 1 cycle latency.
   return DefMI->isTransient() ? 0 : TII->defaultDefLatency(SchedModel, DefMI);
 }