aco: optimize swizzled SALU 8/16-bit conversions

aco: optimize swizzled SALU 8/16-bit conversions

We only need one s_bfe for a conversion with a swizzled source.

shader-db (parallel-rdp, Navi):
Totals from 487 (71.30% of 683) affected shaders:
SpillSGPRs: 3284 -> 3233 (-1.55%); split: -2.71%, +1.16%
SpillVGPRs: 2174 -> 2150 (-1.10%); split: -1.24%, +0.14%
CodeSize: 2497864 -> 2445544 (-2.09%); split: -2.11%, +0.01%
Instrs: 450613 -> 445104 (-1.22%); split: -1.27%, +0.05%

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/5259>