Index: lib/Transforms/Scalar/Sink.cpp
===================================================================
--- lib/Transforms/Scalar/Sink.cpp
+++ lib/Transforms/Scalar/Sink.cpp
@@ -169,8 +169,11 @@
   for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end();
       I != E && SuccToSinkTo == nullptr; ++I) {
     BasicBlock *Candidate = (*I)->getBlock();
-    if ((*I)->getIDom()->getBlock() == Inst->getParent() &&
-        IsAcceptableTarget(Inst, Candidate, DT, LI))
+    // A node should always immediate-dominate its children on the dominator
+    // tree.
+    assert((*I)->getIDom()->getBlock() == Inst->getParent() &&
+           "Broken dominator tree ?");
+    if (IsAcceptableTarget(Inst, Candidate, DT, LI))
       SuccToSinkTo = Candidate;
   }