aco: fix register allocation for subdword instructions on GFX10 Cc: 20.1 <mesa-stable@lists.freedesktop.org> Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com> Reviewed-by: Daniel Schürmann <daniel@schuermann.dev> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5148>