aco: implement 8/16-bit instructions which can be trivially widened

When nir_lower_bit_size becomes more capable, we might want to revert some
of this.

fossil-db (parallel-rdp, Navi):
Totals from 217 (31.77% of 683) affected shaders:
SGPRs: 11320 -> 10200 (-9.89%)
VGPRs: 7156 -> 7364 (+2.91%)
CodeSize: 1453948 -> 1430136 (-1.64%); split: -1.66%, +0.02%
Instrs: 258530 -> 254840 (-1.43%); split: -1.44%, +0.01%
Cycles: 37334360 -> 37247936 (-0.23%); split: -0.26%, +0.03%

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/4791>
2 files changed