Index: lib/Transforms/Scalar/LoopRotation.cpp
===================================================================
--- lib/Transforms/Scalar/LoopRotation.cpp
+++ lib/Transforms/Scalar/LoopRotation.cpp
@@ -445,6 +445,11 @@
           DomTreeNode *Node = HeaderChildren[I];
           BasicBlock *BB = Node->getBlock();
 
+          // The new header idom has been updated correctly. It by definition
+          // can only be dominated by the preheader.
+          if (BB == NewHeader)
+            continue;
+
           pred_iterator PI = pred_begin(BB);
           BasicBlock *NearestDom = *PI;
           for (pred_iterator PE = pred_end(BB); PI != PE; ++PI)