Fix build warnings

src/IceTargetLowering.h

 //===- subzero/src/IceTargetLowering.h - Lowering interface -----*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file declares the TargetLowering and LoweringContext
@@ skipping 90 matching lines @@
       break;
     case Opt_0:
       translateO0();
       break;
     case Opt_1:
       translateO1();
       break;
     case Opt_2:
       translateO2();
       break;
-    default:
-      Func->setError("Target doesn't specify lowering steps.");
-      break;
     }
   }
   virtual void translateOm1() {
     Func->setError("Target doesn't specify Om1 lowering steps.");
   }
   virtual void translateO0() {
     Func->setError("Target doesn't specify O0 lowering steps.");
   }
   virtual void translateO1() {
     Func->setError("Target doesn't specify O1 lowering steps.");
@@ skipping 137 matching lines @@
   virtual void lower(const VariableDeclaration &Var) = 0;
 
 protected:
   TargetGlobalInitLowering(GlobalContext *Ctx) : Ctx(Ctx) {}
   GlobalContext *Ctx;
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERING_H