implement the null function for the Mips32 subzero compiler

src/IceTargetLoweringMIPS32.h

Index: src/IceTargetLoweringMIPS32.h
diff --git a/src/IceTargetLoweringMIPS32.h b/src/IceTargetLoweringMIPS32.h
index f872af0f35a059cf4854ed83f479e0c8af15e40f..a72c8dbfb388658bfaf39a67aea6cdbe4fabe1fd 100644
--- a/src/IceTargetLoweringMIPS32.h
+++ b/src/IceTargetLoweringMIPS32.h
@@ -76,6 +76,9 @@ public:
     (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;