SystemZ: Avoid implicit iterator conversions, NFC

Avoid implicit conversions from MachineInstrBundleIterator to
MachineInstr* in the SystemZ backend, mainly by preferring MachineInstr&
over MachineInstr* and using range-based for loops.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275137 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed