target/s390x: Finish implementing RISBGN

target/s390x: Finish implementing RISBGN

We added the entry to insn-data.def, but failed to update op_risbg
to match.  No need to special-case the imask inversion, since that
is already ~0 for RISBG (and now RISBGN).

Fixes: 375ee58bedcda359011fe7fa99e0647f66f9ffa0
Fixes: https://bugs.launchpad.net/qemu/+bug/1701798 (s390x part)
Signed-off-by: Richard Henderson <[email protected]>
Message-Id: <20171107145546[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Tested-by: Peter Maydell <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>