[SystemZ] Handle extensions in RxSBG optimizations

The input to an RxSBG operation can be narrower as long as the upper bits
are don't care.  This fixes a FIXME added in r192783.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192790 91177308-0d34-0410-b5e6-96231b3b80d8