aco: fix waiting for scalar stores before "writing back" data on GFX8-GFX9

aco: fix waiting for scalar stores before "writing back" data on GFX8-GFX9

Seems required also on GFX8-GFX9 to achieve correct behaviour. This
is an undocumented behaviour but it makes real sense to me.

pipeline-db on GFX9:
Totals from affected shaders:
SGPRS: 1018 -> 1018 (0.00 %)
VGPRS: 516 -> 516 (0.00 %)
Code Size: 40516 -> 40636 (0.30 %) bytes
Max Waves: 280 -> 280 (0.00 %)

This fixes some sort of sun flickering with Assassins Creed Origins.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2488
Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3750>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3750>