I think it's just sliding up the neck rapidly maybe on the B string
Daniel is exactly right. And remember that it's Dave Mustaine's solo and he never used a Floyd Rose (except on rare occasions like Wake Up Dead). If you watch the video to this song you can see him replicate it.
Start really low, nearer the headstock and slide up to a high note, let go.. repeat the technique again but each time make the note that you slide to lower.. so it sounds like the laugh is lowering in pitch