BEFORE the censors cracked down in mid-1934, Hollywood unleashed a flood of sexy, taboo-busting movies to lure audiences during the Great Depression. Six elusive titles -- most have never been shown on TV -- turn up in "The Pre-Code Hollywood Collection," out tomorrow, which vividly shows just how far the financially struggling studios were willing to push the envelope. Paramount especially seems to have gone out of its way to flout the Production Code Administration (PCA), whose myriad prohibitions weren't seriously enforced until Catholic groups threatened a boycott.