ac: fix ac_build_is_helper_invocation when postponed_kill is null

ac: fix ac_build_is_helper_invocation when postponed_kill is null

If there was no demote() in the shader, ac_build_is_helper_invocation
behaves exactly the same as ac_build_load_helper_invocation, i.e.
the helper lanes are the same as they were at the beginning of the shader.

Fixes: de57ea2a3da ("amd/llvm: implement nir_intrinsic_demote(_if) and nir_intrinsic_is_helper_invocation")
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4301>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4301>