nir/linker: pack varyings with different interpolation qualifier

nir/linker: pack varyings with different interpolation qualifier

Driver like radeonsi load varying in a scalar manner, so prefer to pack
varying with different interpolation qualifier into same slot to save
space.

But driver like panfrost/bifrost can load varying in vector manner,
so prefer to pack varying with same interpolation qualifier.

Driver can add interpolation qualifiers which are able to be
packed into same varying slot to pack_varying_options nir option.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12537>