[turbofan] Optimize silent hole checks on legacy const context slots.

src/compiler/common-operator-reducer.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_OPERATOR_REDUCER_H_
 #define V8_COMPILER_COMMON_OPERATOR_REDUCER_H_
 
 #include "src/compiler/graph-reducer.h"
 
 namespace v8 {
 namespace internal {
+
+// Forward declarations.
+class Factory;
+
+
 namespace compiler {
 
 // Forward declarations.
 class CommonOperatorBuilder;
 class Graph;
 class JSGraph;
 class MachineOperatorBuilder;
 class Operator;
 
 
 // Performs strength reduction on nodes that have common operators.
 class CommonOperatorReducer FINAL : public Reducer {
  public:
   explicit CommonOperatorReducer(JSGraph* jsgraph) : jsgraph_(jsgraph) {}
   ~CommonOperatorReducer() FINAL {}
 
   Reduction Reduce(Node* node) FINAL;
 
  private:
   Reduction ReduceEffectPhi(Node* node);
   Reduction ReducePhi(Node* node);
   Reduction ReduceSelect(Node* node);
 
   Reduction Change(Node* node, Operator const* op, Node* a);
   Reduction Change(Node* node, Operator const* op, Node* a, Node* b);
 
   CommonOperatorBuilder* common() const;
   Graph* graph() const;
+  Factory* factory() const;
   JSGraph* jsgraph() const { return jsgraph_; }
   MachineOperatorBuilder* machine() const;
 
   JSGraph* const jsgraph_;
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_COMMON_OPERATOR_REDUCER_H_