ADT: Stop using getNodePtrUnchecked on end() iterators

Stop using `getNodePtrUnchecked()` when building IR.  Eventually a
dereference will be required to get at the downcast node, since the
iterator will only store an `ilist_node_base` of some sort.

This should have no functionality change for now, but is a path towards
removing some more UB from ilist.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261495 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/BasicBlock.cpp

@@ -361,10 +361,8 @@ BasicBlock *BasicBlock::splitBasicBlock(iterator I, const Twine &BBName) {
   assert(I != InstList.end() &&
          "Trying to get me to create degenerate basic block!");
 
-  BasicBlock *InsertBefore = std::next(Function::iterator(this))
-                               .getNodePtrUnchecked();
-  BasicBlock *New = BasicBlock::Create(getContext(), BBName,
-                                       getParent(), InsertBefore);
+  BasicBlock *New = BasicBlock::Create(getContext(), BBName, getParent(),
+                                       this->getNextNode());
 
   // Save DebugLoc of split point before invalidating iterator.
   DebugLoc Loc = I->getDebugLoc();