[turbofan] Clean up cached nodes in JSGraph.

src/compiler/js-graph.h

Index: src/compiler/js-graph.h
diff --git a/src/compiler/js-graph.h b/src/compiler/js-graph.h
index fe9b9f7b7ccdd0aa8e7e436128c64a3ea1d399f5..13dc367d4dd6cab8440fcd0074e6947d430a0644 100644
--- a/src/compiler/js-graph.h
+++ b/src/compiler/js-graph.h
@@ -30,7 +30,9 @@ class JSGraph : public ZoneObject {
         common_(common),
         javascript_(javascript),
         machine_(machine),
-        cache_(zone()) {}
+        cache_(zone()) {
+    for (int i = 0; i < kNumCachedNodes; i++) cached_nodes_[i] = nullptr;
+  }
 
   // Canonicalized global constants.
   Node* CEntryStubConstant(int result_size);
@@ -128,26 +130,28 @@ class JSGraph : public ZoneObject {
   void GetCachedNodes(NodeVector* nodes);
 
  private:
+  enum CachedNode {
+    kCEntryStubConstant,
+    kUndefinedConstant,
+    kTheHoleConstant,
+    kTrueConstant,
+    kFalseConstant,
+    kNullConstant,
+    kZeroConstant,
+    kOneConstant,
+    kNaNConstant,
+    kEmptyFrameState,
+    kDeadControl,
+    kNumCachedNodes  // Must remain last.
+  };
+
   Isolate* isolate_;
   Graph* graph_;
   CommonOperatorBuilder* common_;
   JSOperatorBuilder* javascript_;
   MachineOperatorBuilder* machine_;
-
-  // TODO(titzer): make this into a simple array.
-  SetOncePointer<Node> c_entry_stub_constant_;
-  SetOncePointer<Node> undefined_constant_;
-  SetOncePointer<Node> the_hole_constant_;
-  SetOncePointer<Node> true_constant_;
-  SetOncePointer<Node> false_constant_;
-  SetOncePointer<Node> null_constant_;
-  SetOncePointer<Node> zero_constant_;
-  SetOncePointer<Node> one_constant_;
-  SetOncePointer<Node> nan_constant_;
-  SetOncePointer<Node> empty_frame_state_;
-  SetOncePointer<Node> dead_control_;
-
   CommonNodeCache cache_;
+  Node* cached_nodes_[kNumCachedNodes];
 
   Node* ImmovableHeapConstant(Handle<HeapObject> value);
   Node* NumberConstant(double value);