nir/clip_disable: handle 2x vec4 case

some drivers may have pre-lowered gl_ClipDistance to 2x vec4 to match hw
usage, so for those cases we'll be getting deref_var here and then components
will be stored to the deref at some point

fixes mesa/mesa#3480

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6563>
1 file changed