aco: Implement subgroup shuffle on GFX6-7.

aco: Implement subgroup shuffle on GFX6-7.

GFX6 and GFX7 don't have the ds_bpermute (or permute) instruction,
but we would like to support subgroup shuffle on these old GPUs.

So we introduce a new pseudio instruction which will be lowered
to an "unrolled loop" that emulates bpermute on GFX6 and GFX7
using readlane instructions, while also respecting the exec mask
thanks to v_cmpx.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5223>