Silence GCC warning about falling off the end of a non-void function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167618 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/MachineScheduler.cpp b/lib/CodeGen/MachineScheduler.cpp
index de16932..a4817d0 100644
--- a/lib/CodeGen/MachineScheduler.cpp
+++ b/lib/CodeGen/MachineScheduler.cpp
@@ -1583,6 +1583,7 @@
   case NextDefUse:     return "DEF-USE   ";
   case NodeOrder:      return "ORDER     ";
   };
+  llvm_unreachable("Unknown reason!");
 }
 
 void ConvergingScheduler::traceCandidate(const SchedCandidate &Cand,