The 'varying fetch' pp instruction deals only with coordinates, and
'texture fetch' deals only with the sampler index.
Previously it was not possible to clearly map ppir_op_load_coords and
ppir_op_load_texture to pp instructions as the source coordinates were
kept in the ppir_op_load_texture node, making this harder to maintain.
The refactor is made with the attempt to clearly map ppir_op_load_coords
to the 'varying fetch' and ppir_op_load_texture to the 'texture fetch'.
The coordinates are still temporarily kept in the ppir_op_load_texture
node as nir has both sampler and coordinates in a single instruction and
it is only possible to output one ppir node during emit. But now after
lowering, the sources are transferred to the (always) created
ppir_op_load_coords node, and it should be possible to directly map them
to their pp instructions from there onwards.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Qiang Yu <yuq825@gmail.com>