aco: use v_cvt_f32_ubyte for signed casts too

aco: use v_cvt_f32_ubyte for signed casts too

The extract is always positive, so signed vs unsigned conversion doesn't matter.

Foz-DB GFX11:
Totals from 167 (0.13% of 133461) affected shaders:
Instrs: 401631 -> 401225 (-0.10%)
CodeSize: 2107256 -> 2104344 (-0.14%)
VGPRs: 13320 -> 13332 (+0.09%)
Latency: 6468063 -> 6467241 (-0.01%)
InvThroughput: 801854 -> 801653 (-0.03%)
Copies: 13926 -> 13927 (+0.01%); split: -0.08%, +0.09%

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24893>