aco: replace extract_vector with copies

aco: replace extract_vector with copies

Helps a small number of small shaders with situations like this:
a = p_create_vector ...
b = p_extract_vector a, 3
and copy propagation can't be done

Totals from affected shaders:
SGPRS: 14304 -> 14416 (0.78 %)
VGPRS: 8716 -> 6592 (-24.37 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 184664 -> 176888 (-4.21 %) bytes
Max Waves: 6260 -> 6260 (0.00 %)
Instructions: 35561 -> 33617 (-5.47 %)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2883>