aco: create 16-bit mad/fma

aco: create 16-bit mad/fma

fossil-db (Navi, fp16 enabled):
Totals from 1 (0.00% of 127638) affected shaders:
CodeSize: 4868 -> 4552 (-6.49%)
Instrs: 956 -> 863 (-9.73%)
Cycles: 3824 -> 3452 (-9.73%)
VMEM: 504 -> 490 (-2.78%)
SMEM: 109 -> 107 (-1.83%)
VClause: 19 -> 20 (+5.26%)
Copies: 54 -> 58 (+7.41%)
PreVGPRs: 43 -> 41 (-4.65%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5245>