[Preprocessing] of AutoGenerator stumbles over two "opposite" references

[sacnl]

The preprocessing runs into an infinite loop if the model contains an AutoGenerator for an entity as follows.

entity A {
  ref1 : B
}
entity B{
  ref2 : A
}

Excluding the reference from the Autogenerator will help. The problem will likely also occur with recursive structures.
The relevant line is line 884 of PreprocessModel.xtend