aco: Fix convert_to_SDWA when instruction has 3 operands.

Previously, when the instruction had 3 operands, this would cause
possible corruption because of writing to sdwa->sel[2].
This was noticed thanks to GCC 10's -Wstringop-overflow warning.

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/6436>
1 file changed