aco: use p_split_vector for nir_op_unpack_half_*

aco: use p_split_vector for nir_op_unpack_half_*

This enables the use of SDWA if possible

Totals from 9933 (7.27% of 136546) affected shaders (RAVEN):
VGPRs: 731764 -> 731772 (+0.00%); split: -0.00%, +0.00%
CodeSize: 90944852 -> 90671472 (-0.30%); split: -0.30%, +0.00%
Instrs: 17881885 -> 17867831 (-0.08%); split: -0.08%, +0.00%
Cycles: 1597904072 -> 1597771260 (-0.01%); split: -0.01%, +0.00%
VMEM: 1702328 -> 1697383 (-0.29%); split: +0.13%, -0.42%
SMEM: 659583 -> 659049 (-0.08%); split: +0.01%, -0.09%
VClause: 318024 -> 318025 (+0.00%); split: -0.00%, +0.00%
SClause: 631670 -> 631707 (+0.01%); split: -0.01%, +0.01%
Copies: 1504107 -> 1504626 (+0.03%); split: -0.01%, +0.04%
PreVGPRs: 683153 -> 683180 (+0.00%)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6777>