Reland of [turbofan] Implicitly emit eager checkpoint at graph building.

src/compiler/checkpoint-elimination.h

 // Copyright 2016 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_CHECKPOINT_ELIMINATION_H_
 #define V8_COMPILER_CHECKPOINT_ELIMINATION_H_
 
 #include "src/compiler/graph-reducer.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
 
 // Performs elimination of redundant checkpoints within the graph.
 class CheckpointElimination final : public AdvancedReducer {
  public:
   explicit CheckpointElimination(Editor* editor);
   ~CheckpointElimination() final {}
 
   Reduction Reduce(Node* node) final;
+
+ private:
+  Reduction ReduceCheckpoint(Node* node);
+  Reduction ReduceReturn(Node* node);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_CHECKPOINT_ELIMINATION_H_