By Andrea Shalal WASHINGTON (Reuters) -U.S. Treasury Secretary Janet Yellen on Tuesday urged Congress to approve $80 billion in funding for the Internal Revenue Service to help the agency reduce a huge backlog of tax returns and allow it to go after $600 billion in unpaid tax bills. “The IRS is under siege. It is suffering from huge underinvestment,” Yellen told a Senate Finance Committee hearing on Treasury’s budget request for fiscal 2023. Yellen said the agency was dealing with massive problems, including a “huge backlog” in working through tax returns, and lacked the personnel needed to ca…