It's illegal in the United States to discriminate on a basis that has nothing to do with the job. So if your job doesn't require people with sharp analytical skills (trucking, for example), it would be illegal to use an IQ test. In programming, I doubt it would be illegal.

The reason for this is that people who wanted to exclude black people or women from their workforce would pick some irrelevant trait that the disadvantaged group would score lower on, and use that as the reason not to hire.