Node indices should be treated as unsigned integers

Node::_idx is declared as node_idx_t (unsigned int) but at several places we cast it to int. For example, in escape.cpp:

uint ni = n->_idx;
[...]
const TypeOopPtr* tinst = t->cast_to_instance_id(ni);

with TypeOopPtr::cast_to_instance_id(int instance_id). This should be fixed to use unsigned int consistently.