Revert "Remove the explicit SDNodeIterator::operator= in favor of the implicit default"

Accidentally committed a few more of these cleanup changes than
intended. Still breaking these out & tidying them up.

This reverts commit r231135.

llvm-svn: 231136
diff --git a/llvm/lib/Target/AArch64/AArch64PBQPRegAlloc.cpp b/llvm/lib/Target/AArch64/AArch64PBQPRegAlloc.cpp
index 5394875..4690177 100644
--- a/llvm/lib/Target/AArch64/AArch64PBQPRegAlloc.cpp
+++ b/llvm/lib/Target/AArch64/AArch64PBQPRegAlloc.cpp
@@ -319,7 +319,7 @@
 
 static bool regJustKilledBefore(const LiveIntervals &LIs, unsigned reg,
                                 const MachineInstr &MI) {
-  const LiveInterval &LI = LIs.getInterval(reg);
+  LiveInterval LI = LIs.getInterval(reg);
   SlotIndex SI = LIs.getInstructionIndex(&MI);
   return LI.expiredAt(SI);
 }