De-duplicate the feature generator between dyninstfeaturegenerator.cpp and flowgraphwithinstructions.cpp

[thomasdullien]

There's a comment about de-duplicating this code in dyininstfeaturegenerator; the current duplication of the code makes it very brittle.

The performance cost of forcing the dyninstfeaturegenerator to construct a Flowgraphwithinstructions instance is nontrivial, but code duplication is evil.