aco/isel: use more s_pack
I saw this in a RGP capture, it's not as common as I would have hoped though.
Foz-DB Navi31
Totals from 13 (0.02% of 79206) affected shaders:
Instrs: 44871 -> 44838 (-0.07%)
CodeSize: 223804 -> 223608 (-0.09%)
Latency: 220186 -> 220191 (+0.00%); split: -0.01%, +0.02%
InvThroughput: 54169 -> 54186 (+0.03%); split: -0.00%, +0.03%
SALU: 5048 -> 5023 (-0.50%)