[turbofan] Implement proper caching of heap constants in the JSGraph.

src/compiler/common-node-cache.h

 // 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.
 
 #ifndef V8_COMPILER_COMMON_NODE_CACHE_H_
 #define V8_COMPILER_COMMON_NODE_CACHE_H_
 
 #include "src/compiler/node-cache.h"
 
 namespace v8 {
 namespace internal {
 
 // Forward declarations.
 class ExternalReference;
+class HeapObject;
+template <typename>
+class Handle;
 
 
 namespace compiler {
 
 // Bundles various caches for common nodes.
 class CommonNodeCache final {
  public:
   explicit CommonNodeCache(Zone* zone) : zone_(zone) {}
   ~CommonNodeCache() {}
 
@@ skipping 15 matching lines @@
     return float64_constants_.Find(zone(), bit_cast<int64_t>(value));
   }
 
   Node** FindExternalConstant(ExternalReference value);
 
   Node** FindNumberConstant(double value) {
     // We canonicalize double constants at the bit representation level.
     return number_constants_.Find(zone(), bit_cast<int64_t>(value));
   }
 
+  Node** FindHeapConstant(Handle<HeapObject> value);
+
   // Return all nodes from the cache.
   void GetCachedNodes(ZoneVector<Node*>* nodes);
 
   Zone* zone() const { return zone_; }
 
  private:
   Int32NodeCache int32_constants_;
   Int64NodeCache int64_constants_;
   Int32NodeCache float32_constants_;
   Int64NodeCache float64_constants_;
   IntPtrNodeCache external_constants_;
   Int64NodeCache number_constants_;
-  Zone* zone_;
+  IntPtrNodeCache heap_constants_;
+  Zone* const zone_;
 
   DISALLOW_COPY_AND_ASSIGN(CommonNodeCache);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_COMMON_NODE_CACHE_H_