Is it a subset?
Yes, it's a subset of artificial intelligence and artificial intelligence is a subset of computer science.
AI is a simulation of human intelligence that is coupled with processes, i.e.,
self learning (neither program nor an agent has the capabilities of acquiring of information and rules for using the information).
reasoning (neither program nor an agent uses the rules to reach approximate conclusions or definite conclusion and self correction.
Are they two separate fields in Computer Science?
I have heard conflicting information. One professor said they are the synonyms.
Today machine learning and artificial intelligence are becoming buzz words all over the media, various applications have been developed, but out of 100%, you find that only 40% is a real application that's considered AI, then the 60% are computational intelligence applications.
Hint: artificial intelligence is the mother of machine learning; algorithms which corrects their mistakes and makes decisions without human help.