[Local] Prevent potential DT assert on insertEdge

[brzycki]

Summary

There exists a corner-case for function TryToSimplifyUncondBranchFromEmptyBlock() in Local.cpp where it's posible to
pass:

DT->insertEdge(BB, BB);

to the dominator tree subsystem. When this happens the DT code (correctly) asserts since a block cannot dominate itself.

The error was discovered when inspecting the code for D37528 .

[brzycki]

Fixed in patch re-try found in D38558.

llvm/lib/Transforms/Utils/Local.cpp

bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
   std::vector<DominatorTree::UpdateType> Updates;
   if (DT)
     for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
       if (*PI == BB)
         continue;
       DominatorTree::UpdateType UT = {DominatorTree::Delete, *PI, BB};
       if (std::find(Updates.begin(), Updates.end(), UT) == Updates.end()) {
         Updates.push_back(UT);
+        // Succ cannot dominate itself.
+        if (*PI != Succ)
           Updates.push_back({DominatorTree::Insert, *PI, Succ});
       }
     }
 
   if (isa<PHINode>(Succ->begin())) {
     // If there is more than one pred of succ, and there are PHI nodes in
     // the successor, then we need to add incoming edges for the PHI nodes
     //
     const PredBlockVector BBPreds(pred_begin(BB), pred_end(BB));