YOU watch the new Mel Brooks movie, "Life Stinks," the way a dog watches a snake - alert and suspicious, waiting for the wrong move that will prove it's as nasty as you suspect.How else could you watch a comedy about homelessness and poverty, full of slapstick and featuring Mr. Brooks himself in the leading role? Brooks has always traded in bad taste, of course, with jokes that aim below the belt rather than above the collar. But surely this goes beyond bad taste: gags about the wretched of Los Angeles, jokes about people who don't know where their next meal is coming from. Amazingly, however, that wrong move never quite arrives, and "Life Stinks" turns out to be one of Brooks's most worthwhile movies. True, it pokes fun at hunger and homelessness. But then, Brooks pokes fun at everything in his pictures, as in his musical-comedy version of the Spanish Inquisition in "History of the World, Part 1" a few years ago. In this film, his humor is far from polite, yet it is never nasty toward the story's most helpless characters. And it serves the excellent purpose of reminding us that urgent social problems are all around us right now, and that as soon as we are through laughing at this movie, we should go home and do some hard thinking about them. The main characters of "Life Stinks" are two rival millionaires who want to level and rebuild a section of L. A., regardless of how many poor people are displaced or endangered by the project. The tycoons decide to settle their real estate rivalry with a wager: Can one of them (the Brooks character, of course) survive in the slums for 30 days without money, a place to live, or any of the upper-class comforts that normally surround him? Living in the street, he makes new friends and learns about a side of life he had never thought about before. In his previous pictures, Brooks has rarely aimed his satire directly at the real world, preferring to parody films made by other people: westerns in "Blazing Saddles," horror movies in "Young Frankenstein," silent movies in "Silent Movie," and so forth. His career takes a new turn in "Life Stinks," which comments on social problems that are not only real but troubling and urgent. At its best, the result is barbed and even provocative, reminding me of work by the great Spanish filmmaker Luis Bunfuel, who also took a sardonic attitude toward both the rich and the poor. Like him, Brooks refuses to sentimentalize the homeless any more than he flatters the wealthy. Everyone is pretty ridiculous in his world, and that is all the more reason why people should treat one another with more fairness, decency, and - just as important - humor. Brooks's accomplices in the farce include Lesley Ann Warren as a shopping-bag woman he falls in love with - the picture's most hilarious gag springs from a movie-musical number they frolic through together - and veteran comedian Howard Morris as demented denizen of the streets. I'm not claiming their efforts add up to a profound look at today's urban problems, but hey, what did you expect from a movie called "Life Stinks"? If it gets moviegoers thinking even a tiny bit harder about the miseries going on outside our local multiplexes, that's certainly worth the price of admission.