[SystemZ] Improve risbg-01.ll test

The old mask in f24 wasn't well chosen because the lshr would always be zero.
CodeGen didn't detect this but InstCombine would.  The new mask ensures
that both shifts are needed.

f26 is specifically testing for a wrap-around mask.  The AND can be applied
to just the shift left, either before or after the shift.  Again, CodeGen
kept it in the original form but InstCombine would mask after the shift
instead.  The exact choice of NILF isn't important for the test so I just
dropped it and kept the rotate.