s390x: Add sthyi cc==0 r2+1 verification

On success r2 + 1 should be 0, let's also check for that.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>