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

tests_lit/llvm2ice_tests/contract.ll

Index: 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 0000000000000000000000000000000000000000..59e6491efb291f22994417ced9cd82850203d8d0
--- /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