Subzero: Make optimizations more resilient for early Target development.

src/IceCfgNode.cpp

Index: src/IceCfgNode.cpp
diff --git a/src/IceCfgNode.cpp b/src/IceCfgNode.cpp
index 9e3f23adf634394616a2804d6da7f79530490304..f06e6312fbfa6341ea4688b5803fcda367767cb4 100644
--- a/src/IceCfgNode.cpp
+++ b/src/IceCfgNode.cpp
@@ -852,10 +852,16 @@ void CfgNode::contractIfEmpty() {
     else if (!I.isRedundantAssign())
       return;
   }
+  // Make sure there is actually a successor to repoint in-edges to.
+  if (OutEdges.empty())
+    return;
   assert(OutEdges.size() == 1);
   // Don't try to delete a self-loop.
   if (OutEdges[0] == this)
     return;
+  // Make sure the node actually contains (ends with) an unconditional branch.
+  if (Branch == nullptr)
+    return;
 
   Branch->setDeleted();
   CfgNode *Successor = OutEdges.front();