Currently group bits are stored in bits 0:7, where operand bits are stored.

Make group bits be 0:3, and move the existing bits 0:3 to 16:19, so we can
mix group and operand bits.
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>