Simplify AST ID generation.

src/ast.h

 // Copyright 2012 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_AST_H_
 #define V8_AST_H_
 
 #include "src/v8.h"
 
 #include "src/assembler.h"
@@ skipping 166 matching lines @@
   int node_count_;
   int feedback_slots_;
 };
 
 
 class AstNode: public ZoneObject {
  public:
   // For generating IDs for AstNodes.
   class IdGen {
    public:
-    explicit IdGen(int id = 0) : id_(id) {}
+    IdGen() : id_(BailoutId::FirstUsable().ToInt()) {}
 
     int GetNextId() { return ReserveIdRange(1); }
     int ReserveIdRange(int n) {
       int tmp = id_;
       id_ += n;
       return tmp;
     }
 
    private:
     int id_;
+
+    DISALLOW_COPY_AND_ASSIGN(IdGen);
   };
 
 #define DECLARE_TYPE_ENUM(type) k##type,
   enum NodeType {
     AST_NODE_LIST(DECLARE_TYPE_ENUM)
     kInvalid = -1
   };
 #undef DECLARE_TYPE_ENUM
 
   void* operator new(size_t size, Zone* zone) {
@@ skipping 3378 matching lines @@
   Zone* zone_;
   Visitor visitor_;
   AstValueFactory* ast_value_factory_;
   AstNode::IdGen* id_gen_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_AST_H_