[turbofan] Introduce an ExternalPointer type.

src/compiler/ast-graph-builder.cc

Index: src/compiler/ast-graph-builder.cc
diff --git a/src/compiler/ast-graph-builder.cc b/src/compiler/ast-graph-builder.cc
index 4db8faf51685466ee2529f8747b3f7928d2b0434..f0b78ed59d59c970e5ebc339da0411afb6257bc7 100644
--- a/src/compiler/ast-graph-builder.cc
+++ b/src/compiler/ast-graph-builder.cc
@@ -3780,7 +3780,7 @@ Node* AstGraphBuilder::BuildReturn(Node* return_value) {
     return_value =
         NewNode(javascript()->CallRuntime(Runtime::kTraceExit), return_value);
   }
-  Node* pop_node = jsgraph()->Int32Constant(0);
+  Node* pop_node = jsgraph()->ZeroConstant();
   Node* control = NewNode(common()->Return(), pop_node, return_value);
   UpdateControlDependencyToLeaveFunction(control);
   return control;