Skip new header dominator tree update in loop rotation

[trentxintong]

Summary

Skip new header dominator tree update in loop rotation.

lib/Transforms/Scalar/LoopRotation.cpp

if (DT) {
                                                    OrigHeaderNode->end());
       bool Changed;
       do {
         Changed = false;
         for (unsigned I = 0, E = HeaderChildren.size(); I != E; ++I) {
           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)
             NearestDom = DT->findNearestCommonDominator(NearestDom, *PI);
 
           // Remember if this changes the DomTree.
           if (Node->getIDom()->getBlock() != NearestDom) {
             DT->changeImmediateDominator(BB, NearestDom);