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

There doesn't seem to be any need to assert that iterator assignment is
between iterators over the same node - if you want to reuse an iterator
variable to iterate another node, that's perfectly acceptable. Just
don't mix comparisons between iterators into disjoint sequences, as
usual.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231135 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/ADT/ilistTest.cpp

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