zink: process non-optimal-key passes first

zink: process non-optimal-key passes first

Right now, it's only the vertex-shader that needs special handling for
non-optimal keys. That makes it possible to use fallthrough to always
end up in the last-vertex-stage conditional.

But we're about to add special handling for the geometry stage as well,
so let's prepare by splitting the switch-statement in two; one that only
happens for non-optimal keys, and does all the needed processing there,
and one that deals with the rest.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19117>