nir: Separate texture from sampler in nir_tex_instr

nir: Separate texture from sampler in nir_tex_instr

This commit adds the capability to NIR to support separate textures and
samplers.  As it currently stands, glsl_to_nir only sets the sampler and
leaves the texture alone as it did before and nir_lower_samplers assumes
this.  However, backends can, if they wish, assume that they are separate
because nir_lower_samplers sets both texture and sampler index (they are
the same in this case).