ac/nir: When loading an arg, assert that it's used

To avoid weird crashes caused by trying to load unused attributes.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Natalie Vock <natalie.vock@gmx.de>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41780>

src/amd/common/nir/ac_nir.c

@@ -182,6 +182,8 @@ nir_def *
 ac_nir_load_arg_at_offset(nir_builder *b, const struct ac_shader_args *ac_args,
                           struct ac_arg arg, unsigned relative_index)
 {
+   assert(arg.used);
+
    unsigned arg_index = arg.arg_index + relative_index;
    unsigned num_components = ac_args->args[arg_index].size;