There's also a write up about the film on Wikipedia with some photos. The "Filming" section references Circus Circus & other casinos in the film.

If I recall correctly, there's a car chase scene in the movie that was spiced together to show the cars downtown, then the Strip, then back downtown, then the Strip again. Rather ridiculous if you know Las Vegas topography.

See:
https://en.wikipedia.org/wiki/Diamonds_Are_Forever_(film)