intel/nir: Stop returning the shader from helpers

Now that NIR_TEST_* doesn't swap the shader out from under us, it's
sufficient to just modify the shader rather than having to return in
case we're testing serialization or cloning.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/gallium/drivers/iris/iris_program.c

@@ -1857,7 +1857,7 @@ iris_create_uncompiled_shader(struct pipe_context *ctx,
    if (!ish)
       return NULL;
 
-   nir = brw_preprocess_nir(screen->compiler, nir, NULL);
+   brw_preprocess_nir(screen->compiler, nir, NULL);
 
    NIR_PASS_V(nir, brw_nir_lower_image_load_store, devinfo);
    NIR_PASS_V(nir, iris_lower_storage_image_derefs);