Search was pretty actively against AI usage at the time I left it in 2014. Much of this was because of Amit Singhal, though: he had a strong belief that search should be debuggable, and if there was a query that didn't work you should be able to look into the scores and understand why. There were AI prototypes that actually gave better results than the existing ranking algorithms, but weren't placed into production because of concerns on maintainability. I have no idea if this changed after Amit left.

I work on Assistant now, since recently rejoining Google, and it uses AI for the usual suspects: speech and NLP.