Make unification "predicate" smarter such that it does not result in an infinite type generator

[Joachim Jansen]

Attached file throws an infinite type generation error from XSB. However, it occurs trying to execute X = 1. During "normal execution", X must bound, so this becomes int(X), X = 1.

However, these cases could be detected such that this error is not generated.

[Bart Bogaerts]

See also #670 and #671.