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. (please correct me if I'm wrong @alyssa)
Driver can add interpolation qualifiers which are able to be packed into same varying slot to pack_varying_options nir option.
Only enable for radeonsi in this MR. Other drivers like i965 and RADV could enable this in following MR if would like to.
v3: also support interpolation type (smooth, noperspective).
v2: switch to use nir option instead of gallium pipe cap.