Don't separate the lhs of exclusive rules unless there's a filtering clause

[Sean Kauffman]

When I fixed the problem with where expressions needing to filter the lhs of exclusive rules, I added some overhead to all exclusive rules. This is a little bit unfortunate, since it doesn’t need to be added to them all. The suggestion here is to check to see if it is necessary to split out the lhs into an atomic rule prior to doing any of that work in the semantic analysis code.

This is a minor performance enhancement, so it is not a high priority.