This is more in line with similar opcodes like umul_32x16.
Also change its const expr: the masking based on bit size was
unnecessary as it is only defined for 32 bits. Use simple casts instead.
Signed-off-by: Job Noorman <jnoorman@igalia.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37863>