Refactor optimize

Refactoring of visitors in optimize.c to simplify (perhaps) and remove boiler plate code. A lot of the visitors uses the exact same code pattern to optimize and check for errors.

In commit baef3a1 a fail compilation test fails but the change should provide a more correct solution. Both lhs and rhs should be checked for errors in a "foo || bar" expression.
Surrounding them with #ifdef DEBUG is not a good solution, as it is not fine-grained. Please just leave them as they were, they work fine that way and are quite useful.