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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231136 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/ADT/ilistTest.cpp

@@ -21,9 +21,7 @@ struct Node : ilist_node<Node> {
   int Value;
 
   Node() {}
-  Node(int Value) : Value(Value) {}
-  Node(const Node&) = default;
-  Node(Node &&RHS) : Value(RHS.Value) { RHS.Value = -1; }
+  Node(int _Value) : Value(_Value) {}
   ~Node() { Value = -1; }
 };