SIL: Avoid dereferencing sentinel nodes in ilist_iterators

The behaviour of ilist has changed in LLVM.  It is no longer permissible to
dereference the `end()` value.  Add a check to ensure that we do not
accidentally dereference the iterator.

lib/SILOptimizer/Transforms/StackPromotion.cpp

@@ -319,7 +319,7 @@ template <> struct GraphTraits<StackPromoter *>
     return nodes_iterator(SP->getFunction()->end(), SP->getFunction()->end());
   }
   static unsigned size(GraphType SP) {
-    return std::distance(nodes_begin(SP), nodes_end(SP));
+    return std::distance(SP->getFunction()->begin(), SP->getFunction()->end());
   }
 };