[turbofan] Improve interplay of ControlReducer and CommonOperatorReducer.

src/compiler/control-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_CONTROL_REDUCER_H_
 #define V8_COMPILER_CONTROL_REDUCER_H_
 
 namespace v8 {
 namespace internal {
 
@@ skipping 12 matching lines @@
  public:
   // Perform branch folding and dead code elimination on the graph.
   static void ReduceGraph(Zone* zone, JSGraph* graph,
                           int max_phis_for_select = 0);
 
   // Reduces a single merge node and attached phis.
   static Node* ReduceMerge(JSGraph* graph, Node* node,
                            int max_phis_for_select = 0);
 
   // Testing interface.
-  static Node* ReducePhiForTesting(JSGraph* graph, Node* node);
   static Node* ReduceIfNodeForTesting(JSGraph* graph, Node* node);
 };
 }
 }
 }  // namespace v8::internal::compiler
 
 #endif  // V8_COMPILER_CONTROL_REDUCER_H_