Label references break `MIR_read_with_func`

[asumagic]

As a workaround to #425, when attempting to use MIR_write_with_func/MIR_read_with_func on code that uses label references (i.e. labels as values), using C repro code similar to #424, I get the following error:

A label not from any function in lref S0_asea_jit_mod0_fn0_entry

If asked, I could try to make a simple C repro.