Fix temporarily invalid instructions in goto convert

[peterschrammel]

See #2356 for the context

A couple of initial suggestions to fix this:

1. Use an explicit dummy target to be passed to make_goto()
   • very simple
   • helps avoiding wrongly incomplete initialisations
   • allows checking that a converted goto program does not have incompletely initialised instructions
2. Refactor goto_convert to perform single-step initialisation of goto instructions
   • quite some work
3. Keep NO_INSTRUCTION_TYPE and move if-then-else optimisation into a goto pass
   • doable, but doesn't feel right
   • makes invalid instructions explicit and the absence of incompletely initialised instructions can be easily checked
4. ?

In any case also https://github.com/diffblue/cbmc/blob/develop/src/goto-programs/goto_program.h#L495 should be removed.

[tautschnig]

Fixed via #2361.