Robustness of structural analysis

A couple of things:

• The structural analysis currently crashes on flow graphs with cycles instead of exiting gracefully.
• 6a56171d allows a "then" or "else" node to terminate the program in an If-Then-Else control structure. The same should be allowed for the "then" node of an If-Then control structure.
• 6a56171d introduces an avoidable ambiguity. It would be better to require either both "leaves" of an If-Then-Else control structure to have a common merge point or to have no successors at all.