When I realized, rather early on, what the “twist” was in this rom-com based on the George Michael song of the same name, I groaned. Really, Emma Thompson, THAT was what you came up with? Thompson, whom I ordinarily love, devised the story with her husband, Greg Wise, and co-wrote the script with Byrony Kimmings. She also has a small, blustery role of an overbearing Yugoslavian mother. The movie is directed by Paul Feig, whose “Bridesmaids” is one of my favorite comedies ever. Yet, “Last Christmas” is cheesy and broad, and it tries sooooo strenuously in every respect that it practically works up a sweat. Emilia Clarke goes merrily over the top as a young woman who is drinking too much, indulging in one-night-stands, couch surfing since she doesn’t have her own place, and working as an elf in a Christmas store. Then, she meets and falls for as a glass-half-full dreamboat played by Henry Goulding, who positively beams. The good news? Um, well, all those George Michael songs on the soundtrack sound throwback fab.