aco: implement 16-bit interp

aco: implement 16-bit interp

For 16-bit bank LDS (ie. Kabini/Stoney) we need a slightly different
path. It's completely untested though because I don't have these
chips but according to vkpipeline-db the generated assembly seems fine.

Note that 16-bit I/O is currently only exposed on GFX9+ for both
compiler backends.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4966>