[mlir] Catch nonconvertible types in async conversion

mlir/lib/Conversion/AsyncToLLVM/AsyncToLLVM.cpp

std::unique_ptr<OperationPass<ModuleOp>> mlir::createConvertAsyncToLLVMPass() {
   return std::make_unique<ConvertAsyncToLLVMPass>();
 }
 
 void mlir::populateAsyncStructuralTypeConversionsAndLegality(
     TypeConverter &typeConverter, RewritePatternSet &patterns,
     ConversionTarget &target) {
   typeConverter.addConversion([&](TokenType type) { return type; });
   typeConverter.addConversion([&](ValueType type) {
-    return ValueType::get(typeConverter.convertType(type.getValueType()));
+    Type converted = typeConverter.convertType(type.getValueType());
+    return converted ? ValueType::get(converted) : converted;
   });
 
   patterns.add<ConvertExecuteOpTypes, ConvertAwaitOpTypes, ConvertYieldOpTypes>(
       typeConverter, patterns.getContext());
 
   target.addDynamicallyLegalOp<AwaitOp, ExecuteOp, async::YieldOp>(
       [&](Operation *op) { return typeConverter.isLegal(op); });
 }