nir: Add un/pack_32_4x8 opcodes

Complement the existing un/pack_32_2x16 opcodes. These are useful for
8-bit format packing. On Midgard, they are equivalent to just a 32-bit
move, but other GPUs could lower to other packs if needed.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <mesa/mesa!5107>
55 jobs for !5107 with pack-4x8 in 15 minutes and 40 seconds (queued for 5 seconds)
merge request