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

src/IceCfgNode.cpp

@@ -748,8 +748,12 @@ void CfgNode::contractIfEmpty() {
     else if (!I.isRedundantAssign())
       return;
   }
-  Branch->setDeleted();
   assert(OutEdges.size() == 1);
+  // Don't try to delete a self-loop.
+  if (OutEdges[0] == this)
+    return;
+
+  Branch->setDeleted();
   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

tests_lit/llvm2ice_tests/contract.ll

+; This tests that an empty node pointing to itself is not contracted.
+; https://code.google.com/p/nativeclient/issues/detail?id=4307
+;
+; RUN: %p2i -i %s --filetype=obj --disassemble --args -O2 \
+; RUN:   | FileCheck %s
+
+define void @SimpleBranch() {
+label0:
+  br label %label2
+label1:
+  br label %label1
+label2:
+  br label %label1
+}
+
+; CHECK-LABEL: SimpleBranch
+; CHECK-NEXT: jmp 0 <SimpleBranch>
+; CHECK-NEXT: hlt