“Hard Truths: The Art of Thornton Dial,” the newly opened exhibition at the High Museum of Art, commands seven galleries and all three floors of the Anne Cox Chambers Wing — a lot of valuable Peachtree Street real estate. Yet the display of 59 wall-mounted assemblages, free-standing sculptures and drawings form such a potent critique of social issues in America, and Dial is such a driven and prolific art-maker, one doesn’t doubt the 20-year retrospective could overtake another three stories.