[turbofan] Value numbering should never replace a node with itself.

src/compiler/value-numbering-reducer-unittest.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <limits>
 
 #include "src/compiler/graph.h"
 #include "src/compiler/value-numbering-reducer.h"
 #include "src/test/test-utils.h"
 
@@ skipping 88 matching lines @@
     r = Reduce(graph()->NewNode(&op1, static_cast<int>(input_count), inputs));
     ASSERT_TRUE(r.Changed());
     EXPECT_EQ(n, r.replacement());
 
     r = Reduce(graph()->NewNode(&op1, static_cast<int>(input_count), inputs));
     ASSERT_TRUE(r.Changed());
     EXPECT_EQ(n, r.replacement());
   }
 }
 
+
+TEST_F(ValueNumberingReducerTest, WontReplaceNodeWithItself) {
+  Node* n = graph()->NewNode(&kOp0);
+  EXPECT_FALSE(Reduce(n).Changed());
+  EXPECT_FALSE(Reduce(n).Changed());
+}
+
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8