This is a big deal for debugging if nothing else ("what class is this
ir_instruction, really?"), but is also nice for avoiding building a
whole visitor or an if (node->as_whatever() || node->as_other_thing())
chain.
There is no setter function, the getter returns a constant pointer,
and ir_function_signature::_function is private for a reason. The
only way to make a connection between a function and function
signature is via ir_function::add_signature. This helps ensure that
certain invariants (i.e., a function signature is in the list of
signatures for its _function) are met.
