target-ppc: fix xscmpodp and xscmpudp decoding

The xscmpodp and xscmpudp instructions only have the AX, BX bits in
there encoding, the lowest bit (usually TX) is marked as an invalid
bit. We therefore can't decode them with GEN_XX2FORM, which decodes
the two lowest bit.

Introduce a new form GEN_XX2FORM, which decodes AX and BX and mark
the lowest bit as invalid.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
