Cleanup using C99.
Do not perform optimization in case of overflows.

This fixes opt/fehler222.c.
iredges: use the same block walking behaviour as irg_block_walk

We need a consistent behaviour between irg_block_walk_graph and the
iredges block walk! This means we have to skip tuples like the
irg_block_walk_graph.

This fixes valgrind errors found in opt/fehler217.c