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.
Add two invariant checks related to functions and function signatures:
1. Ensure that function definitions (ir_function) are not nested.
2. Ensure that the ir_function pointed to by an ir_function_signature
is the one that contains it in its signatures list.
