aco/optimizer: optimize extract(extract())

aco/optimizer: optimize extract(extract())

Totals from 53 (0.04% of 134572) affected shaders: (GFX10.3)
SpillVGPRs: 1780 -> 1776 (-0.22%); split: -0.34%, +0.11%
CodeSize: 968352 -> 963196 (-0.53%); split: -0.55%, +0.02%
Scratch: 180224 -> 178176 (-1.14%)
Instrs: 169800 -> 169158 (-0.38%); split: -0.39%, +0.01%
Latency: 6186064 -> 6141408 (-0.72%); split: -1.16%, +0.44%
InvThroughput: 2605044 -> 2582967 (-0.85%); split: -1.37%, +0.52%
VClause: 4851 -> 4866 (+0.31%); split: -0.16%, +0.47%
SClause: 1744 -> 1746 (+0.11%)
Copies: 42874 -> 42325 (-1.28%); split: -1.40%, +0.12%
Branches: 5762 -> 5765 (+0.05%); split: -0.02%, +0.07%

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13576>