nir: update uses_demote flag in discard_to_demote pass

Otherwise the ctx.ac.postponed_kill will not be allocated.

Fixes: ce87da71e9 ("nir: add pass to lower discard() to demote()")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2662
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4301>

src/compiler/nir/nir_lower_discard_to_demote.c

@@ -50,9 +50,11 @@ nir_lower_discard_to_demote(nir_shader *shader)
             switch (intrin->intrinsic) {
             case nir_intrinsic_discard:
                intrin->intrinsic = nir_intrinsic_demote;
+               shader->info.fs.uses_demote = true;
                break;
             case nir_intrinsic_discard_if:
                intrin->intrinsic = nir_intrinsic_demote_if;
+               shader->info.fs.uses_demote = true;
                break;
             case nir_intrinsic_load_helper_invocation:
                intrin->intrinsic = nir_intrinsic_is_helper_invocation;