Subzero: Don't contract an empty node that branches to itself.

src/IceCfgNode.cpp

 //===- subzero/src/IceCfgNode.cpp - Basic block (node) implementation -----===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 730 matching lines @@
     return;
   Inst *Branch = nullptr;
   for (Inst &I : Insts) {
     if (I.isDeleted())
       continue;
     if (I.isUnconditionalBranch())
       Branch = &I;
     else if (!I.isRedundantAssign())
       return;
   }
+  assert(OutEdges.size() == 1);
+  // Don't try to delete a self-loop.
+  if (OutEdges[0] == this)
+    return;
+
   Branch->setDeleted();
-  assert(OutEdges.size() == 1);
   CfgNode *Successor = OutEdges.front();
   // Repoint all this node's in-edges to this node's successor, unless
   // this node's successor is actually itself (in which case the
   // statement "OutEdges.front()->InEdges.push_back(Pred)" could
   // invalidate the iterator over this->InEdges).
   if (Successor != this) {
     for (CfgNode *Pred : InEdges) {
       for (CfgNode *&I : Pred->OutEdges) {
         if (I == this) {
           I = Successor;
@@ skipping 520 matching lines @@
   InstIntrinsicCall *Inst = InstIntrinsicCall::create(
       Func, 5, Func->makeVariable(IceType_i64), RMWI64Name, Info->Info);
   Inst->addArg(AtomicRMWOp);
   Inst->addArg(Counter);
   Inst->addArg(One);
   Inst->addArg(OrderAcquireRelease);
   Insts.push_front(Inst);
 }
 
 } // end of namespace Ice