aco: Workarounds subdword lowering on GFX6/7

As there are no SDWA instructions, we need to take care not to overwrite
the upper bits of other copy_operation's operands.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5226>
1 file changed