[SystemZ] Optimize fcmp X, 0 in cases where X is also negated

In such cases it's often better to test the result of the negation instead,
since the negation also sets CC.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197032 91177308-0d34-0410-b5e6-96231b3b80d8