implement the null function for the Mips32 subzero compiler

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4167
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1176133004 .

Patch from Reed Kotler <reed.kotler@imgtec.com>.

src/IceTargetLoweringMIPS32.h

diff --git a/src/IceTargetLoweringMIPS32.h b/src/IceTargetLoweringMIPS32.h
index f872af0..a72c8db 100644
--- a/src/IceTargetLoweringMIPS32.h
+++ b/src/IceTargetLoweringMIPS32.h
@@ -76,6 +76,9 @@
     (void)C;
     llvm::report_fatal_error("Not yet implemented");
   }
+  void _ret(Variable *RA, Variable *Src0 = nullptr) {
+    Context.insert(InstMIPS32Ret::create(Func, RA, Src0));
+  }
 
   void lowerArguments() override;
   void addProlog(CfgNode *Node) override;