[WebAssembly] Fix return type of nextByte

[tlively]

Summary

The old return type did not allow for correct error reporting and was
causing a compiler warning.

[sbc100]

lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp

124

I know its not really part of this change, but IIUC this is considered overuse of auto in the llvm style. auto is only supposed to be used in cases where the type would otherwise be redundant (i.e. cast<>, dyn_cast<>, make_unique<> etc).

lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp

 extern "C" void LLVMInitializeWebAssemblyDisassembler() {
   // Register the disassembler for each target.
   TargetRegistry::RegisterMCDisassembler(getTheWebAssemblyTarget32(),
                                          createWebAssemblyDisassembler);
   TargetRegistry::RegisterMCDisassembler(getTheWebAssemblyTarget64(),
                                          createWebAssemblyDisassembler);
 }
 
-static uint8_t nextByte(ArrayRef<uint8_t> Bytes, uint64_t &Size) {
+static int nextByte(ArrayRef<uint8_t> Bytes, uint64_t &Size) {
   if (Size >= Bytes.size())
     return -1;
   auto V = Bytes[Size];
   Size++;
   return V;
 }
 
 static bool nextLEB(int64_t &Val, ArrayRef<uint8_t> Bytes, uint64_t &Size,
bool parseImmediate(MCInst &MI, uint64_t &Size, ArrayRef<uint8_t> Bytes) {
   return true;
 }
 
 MCDisassembler::DecodeStatus WebAssemblyDisassembler::getInstruction(
     MCInst &MI, uint64_t &Size, ArrayRef<uint8_t> Bytes, uint64_t /*Address*/,
     raw_ostream & /*OS*/, raw_ostream &CS) const {
   CommentStream = &CS;
   Size = 0;
   auto Opc = nextByte(Bytes, Size);
   if (Opc < 0)
     return MCDisassembler::Fail;
   const auto *WasmInst = &InstructionTable0[Opc];
   // If this is a prefix byte, indirect to another table.
   if (WasmInst->ET == ET_Prefix) {
     WasmInst = nullptr;
     // Linear search, so far only 2 entries.
     for (auto PT = PrefixTable; PT->Table; PT++) {