aco: also reassign p_extract_vector post ra

aco: also reassign p_extract_vector post ra

Foz-DB Navi21:
Totals from 1223 (0.91% of 134864) affected shaders:
CodeSize: 6923888 -> 6913516 (-0.15%)
Instrs: 1293744 -> 1291151 (-0.20%)
Latency: 16928653 -> 16925035 (-0.02%); split: -0.02%, +0.00%
InvThroughput: 2985304 -> 2984775 (-0.02%); split: -0.02%, +0.00%
VClause: 32260 -> 32319 (+0.18%)
SClause: 54952 -> 54949 (-0.01%)
Copies: 83968 -> 81377 (-3.09%)

Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22821>