aco: skip unused channels at the start when fetching vertices

aco: skip unused channels at the start when fetching vertices

pipeline-db (Vega):
Totals from affected shaders:
SGPRS: 161320 -> 161224 (-0.06 %)
VGPRS: 153968 -> 149408 (-2.96 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 4331496 -> 4331308 (-0.00 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 27814 -> 28594 (2.80 %)

pipeline-db (Navi):
Totals from affected shaders:
SGPRS: 161504 -> 161408 (-0.06 %)
VGPRS: 153836 -> 149440 (-2.86 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 4327572 -> 4327604 (0.00 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 27837 -> 28618 (2.81 %)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3086>