Layover in Shanghai heading to Chengdu

Unfortunately, 144 hour visa free transit allows only one stop within mainland China, so you are not eligible for this particular scheme because you have two mainland Chinese destinations in a row.

Another, shorter scheme, called 24 hour visa allows multiple stops within mainland China, but also limits a traveler to just 24 hours counting from the first scheduled arrival into China, until the last scheduled departure from China. If your entire transit time is shorter or equal to exactly 24 hours, you are eligible for 24 hours visa free transit, but if this is not a case, you are not.

Having in mind that your itinerary uses 20 hours of layover in Shanghai, I doubt that with your flight to Chengdu and then with a layover in Chengdu, you cane be within 24 hours time frame... Calculate carefully and if the entire transit time is longer than 24 hours, even for just one minute, you need a Chinese transit visa obtained in advance... Or to alter your itinerary to be in accordance with one of available visa free transit schemes.