nir: Make nir_copy_deref follow the "clone" pattern

We rename it to nir_deref_clone, re-order the sources to match the other
clone functions, and expose nir_deref_var_clone.  This past part, in
particular, lets us get rid of quite a few lines since we no longer have
to call nir_copy_deref and wrap it in deref_as_var.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/intel/vulkan/anv_nir_lower_input_attachments.c

@@ -86,8 +86,7 @@ try_lower_input_load(nir_function_impl *impl, nir_intrinsic_instr *load)
    tex->is_array = true;
    tex->is_shadow = false;
 
-   tex->texture =
-      nir_deref_as_var(nir_copy_deref(tex, &load->variables[0]->deref));
+   tex->texture = nir_deref_var_clone(load->variables[0], tex);
    tex->sampler = NULL;
    tex->texture_index = 0;
    tex->sampler_index = 0;