radv: enable lower_sub to fix loop unrolling.

radv: enable lower_sub to fix loop unrolling.

Loop unroll asserts if it hits a sub, we don't really want
to lower subs as llvm handles these things, but do this for
now, until we can fix loop unroll to work with subs.

Fixes: 14ae0bfa5 (radv: Add NIR loop unrolling)
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>