[WebAssembly][NFC] Delete type checking logic for removed ref.null instruction

[asb]

Summary

D114979 changed the textual formal of ref.null - dropping ref.null in favour of ref.null_extern and ref.null_func. Therefore, the type checker no longer needs logic to handle "ref.null".

[asb]

This patch is somewhat smaller / more straight-forward than I'd usually consider to require pre-commit code review. But I wouldn't mind a quick check in case I'd missed anything about the ref.null => ref.null_{extern,func) transition (e.g. if it is meant to be temporary or something of that nature).

[pmatos]

lgtm, thanks.

llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmTypeCheck.cpp

if (Name == "local.get") {
     const auto *WasmSym = cast<MCSymbolWasm>(&SymRef->getSymbol());
     const auto *Sig = WasmSym->getSignature();
     if (!Sig || WasmSym->getType() != wasm::WASM_SYMBOL_TYPE_TAG)
       return typeError(ErrorLoc, StringRef("symbol ") + WasmSym->getName() +
                                      " missing .tagtype");
     // catch instruction pushes values whose types are specified in the tag's
     // "params" part
     Stack.insert(Stack.end(), Sig->Params.begin(), Sig->Params.end());
-  } else if (Name == "ref.null") {
-    auto VT = static_cast<wasm::ValType>(Inst.getOperand(0).getImm());
-    Stack.push_back(VT);
   } else if (Name == "unreachable") {
     Unreachable = true;
   } else {
     // The current instruction is a stack instruction which doesn't have
     // explicit operands that indicate push/pop types, so we get those from
     // the register version of the same instruction.
     auto RegOpc = WebAssembly::getRegisterOpcode(Opc);
     assert(RegOpc != -1 && "Failed to get register version of MC instruction");