CodeGen: Change MachineInstr to use MachineInstr&, NFC

Change MachineInstr API to prefer MachineInstr& over MachineInstr*
whenever the parameter is expected to be non-null.  Slowly inching
toward being able to fix PR26753.

llvm-svn: 262149
diff --git a/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp b/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
index 93e0ac4..09f7a49 100644
--- a/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
+++ b/llvm/lib/Target/ARM/Thumb1FrameLowering.cpp
@@ -637,7 +637,7 @@
         Reg = ARM::PC;
         (*MIB).setDesc(TII.get(ARM::tPOP_RET));
         if (MI != MBB.end())
-          MIB.copyImplicitOps(&*MI);
+          MIB.copyImplicitOps(*MI);
         MI = MBB.erase(MI);
       } else
         // LR may only be popped into PC, as part of return sequence.