zink: use lowered io (kinda) for i/o vars

zink: use lowered io (kinda) for i/o vars

this runs io lowering on shader create, which is a huge pita
and waste of time since it requires then re-creating all the deref io later,
but it also makes the variables simpler by eliminating struct awfulness
(which was already eliminated by the split_blocks pass, but who's keeping
track) and will enable future use of some bizarro inter-stage linker thing

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24634>