Widen on the local iteration count, as the C++ iterator does

[arthaud]

MonotonicFixpointIterator::extrapolate is documented to apply the join at the first iteration and the widening operator at every subsequent one, which is what MonotonicFixpointIteratorBase::extrapolate does in the C++ implementation:

if (context.get_local_iterations_for(node) == 0) {
  current_state->join_with(new_state);
} else {
  current_state->widen_with(new_state);
}

It read the global iteration count instead, which is never reset. run already calls reset_local_iteration_count_for whenever a component stabilizes, exactly as the C++ iterator does, but nothing read the count back, so a component that stabilized and was then entered again with a larger state was widened right away rather than joined once more.

The result is still a sound post-fixpoint, but it loses precision on nested loops, where the inner component is re-entered every time the outer one grows.

The new test covers this: its inner component stabilizes once before the outer one grows x, so its head is extrapolated with a local iteration count of 0 and a global one of 1. It fails before this change, with z widened to top instead of holding {0, 5}.