[gn] Enable stricter build flags

src/interpreter/bytecodes.h

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_INTERPRETER_BYTECODES_H_
 #define V8_INTERPRETER_BYTECODES_H_
 
 #include <cstdint>
 #include <iosfwd>
 #include <string>
@@ skipping 771 matching lines @@
       case OperandType::kRegOutPair:
         return 2;
       case OperandType::kRegOutTriple:
         return 3;
       case OperandType::kRegList:
         UNREACHABLE();
         return 0;
       default:
         return 0;
     }
+    UNREACHABLE();
     return 0;
   }
 
   // Returns the size of |operand| for |operand_scale|.
   static OperandSize SizeOfOperand(OperandType operand, OperandScale scale);
 
   // Returns true if |operand_type| is a runtime-id operand (kRuntimeId).
   static bool IsRuntimeIdOperandType(OperandType operand_type);
 
   // Returns true if |operand_type| is unsigned, false if signed.
@@ skipping 50 matching lines @@
 };
 
 V8_EXPORT_PRIVATE std::ostream& operator<<(std::ostream& os,
                                            const Bytecode& bytecode);
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_INTERPRETER_BYTECODES_H_