Agenda For Congress --- NPR's Elizabeth Arnold reports on the upcoming Congressional session. Tax reform and affirmative action look to be the most debated topics in 1998. Republicans will be trying hard to simplify the cumbersome tax code and Democrats will be happy to sit back and watch. Both parties say they'll focus on a easier target...the Internal Revenue Service itself. Experts also predict only discussion on the thorny issue of affirmative action in the next year.