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

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

Index: src/compiler/value-numbering-reducer-unittest.cc
diff --git a/src/compiler/value-numbering-reducer-unittest.cc b/src/compiler/value-numbering-reducer-unittest.cc
index a1b543561a2a47251c6378b87df8eee91f6c95d0..28a985605966c1240e131642ce51afff60585526 100644
--- a/src/compiler/value-numbering-reducer-unittest.cc
+++ b/src/compiler/value-numbering-reducer-unittest.cc
@@ -106,6 +106,13 @@ TEST_F(ValueNumberingReducerTest, SubsequentReductionsYieldTheSameNode) {
   }
 }
 
+
+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