aco: use v_cvt_pk_u8_f32 for f2u8

Split from !28882 (merged).

Foz-DB Navi31:
Totals from 42 (0.05% of 79395) affected shaders:
Instrs: 3253747 -> 3248867 (-0.15%); split: -0.15%, +0.00%
CodeSize: 16690136 -> 16661772 (-0.17%); split: -0.17%, +0.00%
VGPRs: 4176 -> 4128 (-1.15%)
Latency: 18485157 -> 18479752 (-0.03%); split: -0.03%, +0.00%
InvThroughput: 3659404 -> 3658222 (-0.03%); split: -0.03%, +0.00%
Copies: 231891 -> 228145 (-1.62%)
VALU: 1785800 -> 1782054 (-0.21%)