[SystemZ] Use "for (auto" a bit

Just the simple cases for now.  There were a few knock-on changes of
MachineBasicBlock *s to MachineBasicBlock &s.  No functional change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203105 91177308-0d34-0410-b5e6-96231b3b80d8