I saw an early preview, and "Boom! Crunch" was the song then (I still have the playbill ... somewhere). But unless I'm wrong, it was really just a draft of "Last Midnight." I don't think "Midnight" was a brand new song, just a refurbished one freshly titled.
And sidebar: the number is one of the most problematic songs in all of Sondheim. It was especially perplexing in the Vanessa Williams revival, when the Witch suddenly grabbed the Bakers' baby and began singing to it, while they were all still talking about "him" and "he" being Jack. The song is about whether to sacrifice Jack to the lady giant, and yet there's the Witch suddenly wondering whether to kidnap this baby. In the middle she thrusts the baby back in the Baker's hands and basically goes back to singing about the Jack dilemma. Very confusing, and still not a good match of action with lyrics. And the song is built around the Witch referencing her own mother, even exiting the world to re-join her. An offstage character mentioned once before, quickly, in the opening scene, when the beans are explained. The Sondheim lyric is haunting, but underneath full of a kind of arch psycholobabble that doesn't help the story telling. And "I'm the witch, I'm the hitch..." line shows the strain. She's explaining her role in this story, and all stories with witches -- sort of providing Cliff Notes on herself as a stock character. That said, when sung well, if confusing dramatically it's still far from boring in context.
"I'm a comedian, but in my spare time, things bother me." Garry Shandling