Fix scheduling with Memorize

Fix issue #209 (closed).

@cguillon I wanted to use backEdge here, to avoid having a specific MemorizeOp case, but figured that it is not exactly what I thought!

I would have expected the back edge to be the second output of the MemorizeOp, which is allowed to form a cycle (i.e. be connected backwards), contrary to its first output. So I don't understand the logic of specifying the input of the MemorizeOp as a back edge, as it does not directly form a cycle?