You're talking about automatic parallelization. There's been quite a bit of research done in this area, but the results aren't that great. Some languages (particularly functional ones) are better suited to automatic parallelization than others, and it's pretty doubtful that you'll see a good automatically parallelizing compiler anytime soon, especially for an imperative interpreted language like Javascript.

See http://en.wikipedia.org/wiki/Automatic_parallelization for more info.