Wrong scheduling because of the lack of memory dependencies about glow

[qcolombet]

After talking with @nadavrot this morning, we are going to explicit state that save nodes happen after the last use of the related variable. In other words, referencing a variable will always get you the old value.

Similarly we are going to enforce that every variable has at most one save nodes. That way it will remove the ambiguity of which save should happen last (all the others would have been dead because of the other restriction.)

[qcolombet]

Fixed by #1307.

[qcolombet]

Problem illustrated in #1284.

This PR adds two test cases that build exactly the same graph. However, by just changing the order of the creation of the nodes, we can trick the scheduler to produce an invalid schedule.

[qcolombet]

As expected while reviewing #1176, I realize that our implicit memory dependencies are going to bite us again during graph partitioning.

The problem there is that the post order visiting methods don't see the memory dependencies edges. So, we will have to do one of the following fix the visiting scheme, come up with targeted fix for the graph partitioning stuff, or model the memory dependencies.