[turbofan] Introduce an ExternalPointer type.

src/compiler/ast-graph-builder.cc

 // 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.
 
 #include "src/compiler/ast-graph-builder.h"
 
 #include "src/ast/compile-time-value.h"
 #include "src/ast/scopes.h"
 #include "src/compilation-info.h"
 #include "src/compiler.h"
@@ skipping 3762 matching lines @@
   return call;
 }
 
 
 Node* AstGraphBuilder::BuildReturn(Node* return_value) {
   // Emit tracing call if requested to do so.
   if (FLAG_trace) {
     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;
 }
 
 
 Node* AstGraphBuilder::BuildThrow(Node* exception_value) {
   NewNode(javascript()->CallRuntime(Runtime::kReThrow), exception_value);
   Node* control = NewNode(common()->Throw(), exception_value);
   UpdateControlDependencyToLeaveFunction(control);
@@ skipping 565 matching lines @@
     // Phi does not exist yet, introduce one.
     value = NewPhi(inputs, value, control);
     value->ReplaceInput(inputs - 1, other);
   }
   return value;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8