aco: try to better align 8+ dword SGPR vectors

aco: try to better align 8+ dword SGPR vectors

This doesn't have much of an effect, but it helps avoid a
pathological case for Assassin's Creed Valhalla and a RDR2 shader with a
future change.

fossil-db (Sienna):
Totals from 55074 (39.51% of 139391) affected shaders:
SGPRs: 3515076 -> 3567744 (+1.50%); split: -0.01%, +1.51%
CodeSize: 206942120 -> 206941868 (-0.00%); split: -0.00%, +0.00%
Instrs: 39625900 -> 39625837 (-0.00%); split: -0.00%, +0.00%
Cycles: 1640088780 -> 1640088828 (+0.00%); split: -0.00%, +0.00%

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4070
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8416>