ac/nir: Slightly refactor how pos0 exports are added when missing.

Prepares for a workaround. Makes it possible for this function
to not emit the pos0 export at all so that it can be emitted
by a subsequent call to the function later.

Cc: mesa-stable
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
(cherry picked from commit 9c096e4a)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25157>