After solving a corner case in bcdsub, this patch simplifies the logic
of both bcdadd/sub instructions by removing some unnecessary local flags.
This commit also rearranges some if-else conditions in bcdadd to make it
easier to read.

Signed-off-by: Yasmin Beatriz <yasmins@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>