aco: fix MUBUF VS input loads when expanding vec3 to vec4 on GFX6

aco: fix MUBUF VS input loads when expanding vec3 to vec4 on GFX6

When some unused channels are skipped and that we expand vec3 loads
to vec4 loads, we have to adjust the fourth component.

While we are at it, add an assertion to make sure we don't use
MUBUF for vec3 loads on GFX6.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2450
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2442
Fixes: 6aecc316 ("aco: fix VS input loads with MUBUF on GFX6")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3641>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3641>