Mel Brooks’ 1967 comic masterpiece is going to be showcased, along with Brooks himself,  this April at the annual TCM Classic Film Festival. Meanwhile, or in case you’re not traveling to Hollywood for the occasion, TCM tonight presents The Producers in prime time, with all of its lunacy, irreverence and utter brilliance intact. If you can’t sing along with every verse of “Springtime for Hitler,” it’s a skill that’s never too late to be acquired…