aco: don't expand vec3 VS input load to vec4 on GFX6

aco: don't expand vec3 VS input load to vec4 on GFX6

Removes the (small) possibility of invalid memory access.

fossil-db (pitcairn):
Totals from 35456 (26.15% of 135610) affected shaders:
MaxWaves: 259508 -> 260642 (+0.44%); split: +0.44%, -0.01%
Instrs: 7915383 -> 7965774 (+0.64%); split: -0.09%, +0.72%
CodeSize: 37163748 -> 37524804 (+0.97%); split: -0.04%, +1.01%
SGPRs: 1515128 -> 1513576 (-0.10%); split: -0.27%, +0.17%
VGPRs: 1218376 -> 1211160 (-0.59%); split: -0.71%, +0.12%
SpillSGPRs: 1152 -> 1144 (-0.69%)
Latency: 83777626 -> 83867137 (+0.11%); split: -0.61%, +0.72%
InvThroughput: 25722445 -> 25727745 (+0.02%); split: -0.23%, +0.25%
VClause: 232058 -> 230464 (-0.69%); split: -2.53%, +1.84%
SClause: 322579 -> 322108 (-0.15%); split: -0.76%, +0.61%
Copies: 547032 -> 547954 (+0.17%); split: -1.83%, +2.00%
Branches: 72538 -> 72542 (+0.01%)
PreVGPRs: 898453 -> 897584 (-0.10%); split: -0.13%, +0.03%

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18225>