Subzero: Fix build warnings/errors under g++.

src/IceAssemblerARM32.cpp

 //===- subzero/src/IceAssemblerARM32.cpp - Assembler for ARM32 --*- C++ -*-===//
 //
 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // Modified by the Subzero authors.
 //
 //===----------------------------------------------------------------------===//
 //
@@ skipping 137 matching lines @@
   case OperandARM32::LSL:
     return 0; // 0b00
   case OperandARM32::LSR:
     return 1; // 0b01
   case OperandARM32::ASR:
     return 2; // 0b10
   case OperandARM32::ROR:
   case OperandARM32::RRX:
     return 3; // 0b11
   }
+
+  llvm::report_fatal_error("Unknown Shift value");
+  return 0;
 }
 
 // Returns the bits in the corresponding masked value.
 inline IValueT mask(IValueT Value, IValueT Shift, IValueT Bits) {
   return (Value >> Shift) & ((1 << Bits) - 1);
 }
 
 // Extract out a Bit in Value.
 inline bool isBitSet(IValueT Bit, IValueT Value) {
   return (Value & Bit) == Bit;
@@ skipping 1014 matching lines @@
     // rr defined (RotationValue) rotate.
     constexpr IValueT Opcode = B26 | B25 | B23 | B22 | B21 | B20;
     emitUxt(Cond, Opcode, Rd, Rn, Rm, Rotation);
     return;
   }
   }
 }
 
 } // end of namespace ARM32
 } // end of namespace Ice