[M68k] Avoid UB in disassembler

[ricky26]

Summary

When reading 32 bits a 32-bit shift would be executed.

This is undefined behaviour, but in this case we can just replace the
entire scratch value to avoid it.

[myhsu]

LGTM Thank you!

llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp

 //===- M68kDisassembler.cpp - Disassembler for M68k -------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file is part of the M68k Disassembler.
 //
 //===----------------------------------------------------------------------===//
 
 #include "M68k.h"
 #include "M68kRegisterInfo.h"
 #include "M68kSubtarget.h"
 #include "MCTargetDesc/M68kMCCodeEmitter.h"
 #include "MCTargetDesc/M68kMCTargetDesc.h"
 #include "TargetInfo/M68kTargetInfo.h"
 
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCContext.h"
case M68kBeads::Imm32:
     break;
   case M68kBeads::Imm3:
     NumToRead = 3;
     break;
   default:
     llvm_unreachable("invalid imm");
   }
 
-  Scratch = (Scratch << NumToRead) | Reader.readBits(NumToRead);
+  Scratch = (NumToRead < 32) ? (Scratch << NumToRead) : 0;
+  Scratch |= Reader.readBits(NumToRead);
 }
 
 DecodeStatus M68kDisassembler::getInstruction(MCInst &Instr, uint64_t &Size,
                                               ArrayRef<uint8_t> Bytes,
                                               uint64_t Address,
                                               raw_ostream &CStream) const {
   // Read and shift the input (fetch as much as we can for now).
   auto Buffer = M68kInstructionBuffer::fill(Bytes);