GeezerVison

I'm not a big fan of itertools.chain. Maybe make this a generator?

I also noticed this is causing the output to be emitted in a different order again. But I quite like having the generated instructions in the same order as they occur in the input. Could you restore that behavior? (I understand that it's a little tricky due to overrides. Ideally there would be a comment left in the output for instructions that are overridden by later ones.)

Okay, I've changed things now so the output will retain the definition order and if there are overrides they'll look something like this:

...
        // TARGET(NOP) overridden by later definition
...
        // Override
        TARGET(NOP) {
            DISPATCH();