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

When not all instructions have a scheduling class,
the error message now provides a possible solution.

Differential Revision: http://reviews.llvm.org/D15854

llvm-svn: 256839

llvm/lib/CodeGen/TargetSchedule.cpp

@@ -212,7 +212,7 @@ unsigned TargetSchedModel::computeOperandLatency(
       && !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