target-ppc: implement branch-less divw[o][.]

While implementing modulo instructions figured out that the
implementation uses many branches. Change the logic to achieve the
branch-less code. Undefined value is set to dividend in case of invalid
input.

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
