[SystemZ] Improve handling of SETCC

We previously used the default expansion to SELECT_CC, which in turn would
expand to "LHI; BRC; LHI".  In most cases it's better to use an IPM-based
sequence instead.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192784 91177308-0d34-0410-b5e6-96231b3b80d8