[SystemZ] Directly use CC result of compare-and-swap

In order to implement a test whether a compare-and-swap succeeded, the
SystemZ back-end currently emits a rather inefficient sequence of first
converting the CC result into an integer, and then testing that integer
against zero.  This commit changes the back-end to simply directly test
the CC value set by the compare-and-swap instruction.

llvm-svn: 322988