[SystemZ] Do not install IfConverter pass at -O0

When not optimizing, do not run the IfConverter pass, this makes
debugging more difficult (and causes a testsuite failure in
DebugInfo/unconditional-branch.ll).



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210263 91177308-0d34-0410-b5e6-96231b3b80d8