Special instrumentation silently fails when instrumenting newly added functions

[ejrgilbert]

We need to either disable instrumentation on newly added functions OR make sure it works in all cases (not just before_at(0)).

To reproduce (silently disregards special instrumentation when encoding):

let global_map_init_fn = FunctionBuilder::new(&[], &[]);
let global_map_init_id = global_map_init_fn.finish_module(app_wasm);

let mut global_map_init = app_wasm.functions.get_fn_modifier(global_map_init_id)
global_map_init.func_entry();
global_map_init.i32_const(0);