IN THE continuing controversy over efforts to restrict immigration, much of the debate has focused on charges that sanctions on the hiring of illegal aliens will cause employers to discriminate against foreign-looking workers in order to avoid breaking the law.  By raising this concern, opponents of immigration restriction have gained the ear of sympathetic liberals, as well as much attention from the media. Their efforts go a long way toward explaining why immigration legislation has been delayed for years and at various points almost killed—though employer sanctions were eventually included in the recently enacted Immigration Reform and Control Act.