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/DenseMapTest.cpp

@@ -46,6 +46,7 @@ public:
   CtorTester(const CtorTester &Arg) : Value(Arg.Value) {
     EXPECT_TRUE(Constructed.insert(this).second);
   }
+  CtorTester &operator=(const CtorTester &) = default;
   ~CtorTester() {
     EXPECT_EQ(1u, Constructed.erase(this));
   }