nvir: Always split 64-bit IMAD/IMUL operations

Those operations do not map to actual hardware instructions, therefore
those should always be lowered to 32-bit instructions.

Fixes: 009c54aa "nv50/ir: Split 64-bit integer MAD/MUL operations"

Signed-off-by: Pierre Moreau <pierre.morrow@free.fr>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
(cherry picked from commit 21b92b34)