Revise "IsEverExecutable" - Make sure it goes as far as it can, without exploding in time

[Kirluu]

Method: Before any other patterns are applied, we apply a graph-global IsEverExecutable check to detect any easily determinable non-executable activities to make their outgoing relations go away.

Motivation: If IsEverExecutable can catch even some false-determinations, then the rest of our patterns (having a lot of constraints on things e.g. never being excluded by others) will be much more likely to also catch redundancies after IsEverExecutable has been run throughout the graph.