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

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

@@ -548,7 +548,6 @@ public:
   }
   PhiState(Value *b) : status(Base), base(b) {}
   PhiState() : status(Unknown), base(nullptr) {}
-  PhiState &operator=(const PhiState &) = default;
   PhiState(const PhiState &other) : status(other.status), base(other.base) {
     assert(status != Base || base);
   }