OR is it the urge to splurge? What a field day Cole Porter and his lyric-writing peers would have had with the latest round in the on-and-off history of big business mergers. Birds do it. Bees do it. First Fidelity and First Union did it. (They'll certainly be true to each other, darling, in their fashion.) Chase was chaste, until it was chased. Then suddenly it found Chemical's chemistry irresistible. That old black Magic Kingdom has Warren Buffet, Tom Murphy, and ABC in its spell. (No wonder - the dowry's $8 billion.) Mating bees buzz. Owls hoot. Western railroads just go on a toot. Even pharmaceutical giants, drugged with love, are worried they'll be late to mate. Somehow, though, the most touching scene of all features Jane Fonda, already merged with Ted Turner, picking up Time Warner's Gerald Levin at a Montana airport to start the biggest merger in all of medialand. Fonda - now there's a name Cole Porter could have run with.