[SystemZ] Optimize X [!=]= Y in cases where X - Y or Y - X is also computed

In those cases it's better to compare the result of the subtraction
against zero.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197239 91177308-0d34-0410-b5e6-96231b3b80d8