Clean up multiply at the same time.

For remainder, generic code will produce mul+sub,
whereas we can implement with msub.
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Reviewed-by: NClaudio Fontana <claudio.fontana@huawei.com>
Tested-by: NClaudio Fontana <claudio.fontana@huawei.com>