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

R=mstarzinger@chromium.org
TEST=compiler-unittests

Review URL: https://codereview.chromium.org/539833003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23724 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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

src/compiler/value-numbering-reducer.cc

@@ -60,6 +60,7 @@ Reduction ValueNumberingReducer::Reduce(Node* node) {
   Entry** head = &buckets_[HashCode(node) % arraysize(buckets_)];
   for (Entry* entry = *head; entry; entry = entry->next()) {
     if (entry->node()->op() == NULL) continue;
+    if (entry->node() == node) return NoChange();
     if (Equals(node, entry->node())) {
       return Replace(entry->node());
     }