radv: assert that the arg is declared when used in get_scalar_arg()

Help debugging.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15789>

src/amd/vulkan/radv_nir_apply_pipeline_layout.c

@@ -42,6 +42,7 @@ typedef struct {
 static nir_ssa_def *
 get_scalar_arg(nir_builder *b, unsigned size, struct ac_arg arg)
 {
+   assert(arg.used);
    return nir_load_scalar_arg_amd(b, size, .base = arg.arg_index);
 }