CodeGen: Use MachineInstr& in LiveVariables API, NFC

Change all the methods in LiveVariables that expect non-null
MachineInstr* to take MachineInstr& and update the call sites.  This
clarifies the API, and designs away a class of iterator to pointer
implicit conversions.

llvm-svn: 274319
diff --git a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
index ed11dcf..2421780 100644
--- a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
@@ -760,7 +760,7 @@
     for (unsigned I = 1; I < NumOps; ++I) {
       MachineOperand &Op = OldMI->getOperand(I);
       if (Op.isReg() && Op.isKill())
-        LV->replaceKillInstruction(Op.getReg(), OldMI, NewMI);
+        LV->replaceKillInstruction(Op.getReg(), *OldMI, *NewMI);
     }
   }
   transferDeadCC(OldMI, NewMI);