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

src/compiler/int64-lowering.h

Index: src/compiler/int64-lowering.h
diff --git a/src/compiler/int64-lowering.h b/src/compiler/int64-lowering.h
index 7a4124794f25c9e6697af007597be794c4b63c4f..34a3ffbd7559fb863a47847f4e75806a84f3f2c3 100644
--- a/src/compiler/int64-lowering.h
+++ b/src/compiler/int64-lowering.h
@@ -47,6 +47,7 @@ class Int64Lowering {
   Node* GetReplacementLow(Node* node);
   bool HasReplacementHigh(Node* node);
   Node* GetReplacementHigh(Node* node);
+  void PreparePhiReplacement(Node* phi);
 
   struct NodeState {
     Node* node;
@@ -58,9 +59,10 @@ class Int64Lowering {
   MachineOperatorBuilder* machine_;
   CommonOperatorBuilder* common_;
   NodeMarker<State> state_;
-  ZoneStack<NodeState> stack_;
+  ZoneDeque<NodeState> stack_;
   Replacement* replacements_;
   Signature<MachineRepresentation>* signature_;
+  Node* placeholder_;
 };
 
 }  // namespace compiler