message: [XLA] Redesign: add comparator and printer for the XlaOp.
  This is to prepare the migration of tf2xla. There were some codes used ComputationDataHandle::handle() for comparison/printing. Now implement XlaOp's comparator and printer.
diff: @@ -57,11 +57,27 @@ class XlaOp {
  StatusOr<Shape> GetShape() const;
  + const XlaBuilder* builder() const { return builder_; }
  +
  + bool operator==(const XlaOp& rhs) const {
  + return handle_ == rhs.handle_ && builder_ == rhs.builder_;
  + }
  +
  + bool operator!=(const XlaOp& rhs) const {
  + return handle_ != rhs.handle_ || builder_ != rhs.builder_;
  + }
  +
  + friend std::ostream& operator<<(std::ostream& out, const XlaOp& op) {
  + out << op.handle();
  + return out;
  + }
  +
  private:
  XlaOp(int64 handle, XlaBuilder* builder)
  : handle_(handle), builder_(builder) {}
  int64 handle() const { return handle_; }
  +
  friend class XlaBuilder;
  int64 handle_;
  