[SystemZ] Optimize floating-point comparisons with zero

This follows the same lines as the integer code.  In the end it seemed
easier to have a second 4-bit mask in TSFlags to specify the compare-like
CC values.  That eats one more TSFlags bit than adding a CCHasUnordered
would have done, but it feels more concise.

llvm-svn: 187883
7 files changed