st/glsl_to_nir: move nir_lower_io to drivers

This allows drivers more freedom in how exactly they want to lower I/O,
e.g. first lowering I/O to temporaries.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/drivers/vc4/vc4_program.c

@@ -2427,6 +2427,9 @@ vc4_shader_state_create(struct pipe_context *pctx,
                  * creation.
                  */
                 s = cso->ir.nir;
+
+                NIR_PASS_V(s, nir_lower_io, nir_var_all, type_size,
+                           (nir_lower_io_options)0);
         } else {
                 assert(cso->type == PIPE_SHADER_IR_TGSI);