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

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4307
R=ascull@google.com

Review URL: https://codereview.chromium.org/1337113008 .

src/IceCfgNode.cpp

diff --git a/src/IceCfgNode.cpp b/src/IceCfgNode.cpp
index d57224b..58e74d4 100644
--- a/src/IceCfgNode.cpp
+++ b/src/IceCfgNode.cpp
@@ -748,8 +748,12 @@
     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

diff --git a/tests_lit/llvm2ice_tests/contract.ll b/tests_lit/llvm2ice_tests/contract.ll
new file mode 100644
index 0000000..59e6491
--- /dev/null
+++ b/tests_lit/llvm2ice_tests/contract.ll
@@ -0,0 +1,18 @@
+; 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