implement the null function for the Mips32 subzero compiler

src/IceTargetLoweringMIPS32.h

 //===- subzero/src/IceTargetLoweringMIPS32.h - MIPS32 lowering ---*- C++-*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 58 matching lines @@
     llvm::report_fatal_error("Not yet implemented");
   }
   void emit(const ConstantFloat *C) const final {
     (void)C;
     llvm::report_fatal_error("Not yet implemented");
   }
   void emit(const ConstantDouble *C) const final {
     (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;
   void addEpilog(CfgNode *Node) override;
 
 protected:
   explicit TargetMIPS32(Cfg *Func);
 
   void postLower() override;
 
@@ skipping 74 matching lines @@
 protected:
   explicit TargetHeaderMIPS32(GlobalContext *Ctx);
 
 private:
   ~TargetHeaderMIPS32() = default;
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERINGMIPS32_H