[wasm] New attempt to implement the Int64Lowering of phis.

src/compiler/int64-lowering.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_INT64_LOWERING_H_
 #define V8_COMPILER_INT64_LOWERING_H_
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/graph.h"
 #include "src/compiler/machine-operator.h"
@@ skipping 29 matching lines @@
   void LowerNode(Node* node);
   bool DefaultLowering(Node* node);
   void LowerComparison(Node* node, const Operator* signed_op,
                        const Operator* unsigned_op);
 
   void ReplaceNode(Node* old, Node* new_low, Node* new_high);
   bool HasReplacementLow(Node* node);
   Node* GetReplacementLow(Node* node);
   bool HasReplacementHigh(Node* node);
   Node* GetReplacementHigh(Node* node);
+  void PreparePhiReplacement(Node* phi);
 
   struct NodeState {
     Node* node;
     int input_index;
   };
 
   Zone* zone_;
   Graph* const graph_;
   MachineOperatorBuilder* machine_;
   CommonOperatorBuilder* common_;
   NodeMarker<State> state_;
-  ZoneStack<NodeState> stack_;
+  ZoneDeque<NodeState> stack_;
   Replacement* replacements_;
   Signature<MachineRepresentation>* signature_;
+  Node* placeholder_;
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_INT64_LOWERING_H_