Minor update to improve graph management and also, the DOT printer

It will be better for futur to save the Rules with the Graph
This modifications allow us to show a better/cleaner DOT file.

Summary: Update a Graph visitor

Test Plan: try the "-M dot" option (output is cleaner)

Reviewers: adrienconrath

Reviewed By: adrienconrath

Subscribers: NicolasDP

Differential Revision: http://91.121.199.76/D21