Silence "not all paths return a value" warning in MSVC

llvm-svn: 227614

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -746,6 +746,7 @@ public:
       case IK_NoInduction:
         return nullptr;
       }
+      llvm_unreachable("invalid enum");
     }
 
     /// Start value.