The public method
void Compile::igv_print_graph_to_network(const char* name, Node* node, GrowableArray<const Node*>& visible_nodes)
doesn't use its second parameter `Node* node`: the implementation uses always `C->root()`, and doesn't allow to print a sub-graph without another root as one may think looking at the signature.

Luckily, all the call-site are providing `C->root()` at the moment, so this bug is unnoticed.

To keep a clean public interface, we should remove this unused parameter. We could also make use of it instead of always using `C->root()` but there seems to be no such need and it'd be inconsistent with `Compile::igv_print_method_to_file` that always prints from root.