The Great Hall at the Metropolitan Museum of Art is such a busy place that it’s easy to miss the artworks periodically on view there. Two 1967-68 silk-screens of Warhol’s “Flowers” — a teaser to “Regarding Warhol: Sixty Artists, Fifty Years,” opening on Tuesday — were recently hung on the eastern side of the hall. A colossal statue of a pharaoh, weighing about nine tons, sits regally on the north side.