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

src/compiler/value-numbering-reducer.cc

Index: src/compiler/value-numbering-reducer.cc
diff --git a/src/compiler/value-numbering-reducer.cc b/src/compiler/value-numbering-reducer.cc
index 4fe4b39c389ffa69bcec22f196730b1b862630bb..bdc9a520174a0c3d89d8123ec819fe8c5580c4d2 100644
--- a/src/compiler/value-numbering-reducer.cc
+++ b/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());
     }