RFC: Explicit move preference hints

[Wilco Dijkstra]

Segher Boessenkool wrote:
>
> "0,r" might work, or "0,?r", or similar (alternatives have commas
> between them).

No, it doesn't work at all. But that is no surprise if you look at ira_get_dup_out_num.
It iterates over the constraint string and if you have anything that matches after a "0",
the "0" constraint is simply ignored. So "0" works only if it is the only constraint,
and any combination with an unconstrained register makes no difference
irrespectively whether it is "0r" or "0, r" or "0, ?!^$r".

So that is why I'd like to fix this so it actually works like you'd expect.

Wilco