Refactoring Associative engine

This PR is refactoring work on the following:
1. Move functions that compute associativity from DSASM.Executive to AssociativeEngine.Utils
2. Refactor UpdateDependencyGraph  such that it only returns the affected graphnodes and only marking them dirty after the call to UpdateDependencyGraph.

This is a step towards cleaning up the assoc engine.
This PR also adds the stubs to implement static analysis on VM graphnodes.

No testcase required as there are semantic changes.

PTAL @ke-yu @lukechurch 
I'd suggest that any function that returns a collection, should really return a collection. If it returns a null, that means some bad thing happen, so that in the code we don't need to check if the collection is null or not. :-)