If the US dollar crashes, bitcoin will most likely crash. This is because most of the bitcoin users are from the US, and with them gone, bitcoin will basically be much, much cheaper, as the americans try to sell their coins to get some sort of cash. Or maybe BTC will go up, they will try to convert their money into bitcoin, and cause a huge price spike.
If the USD devalued - assuming everything else was constant Bitcoin's price compared to USD would actually increase (weakened USD alongside possible increased demand for alternate currencies such as Bitcoin). But in all honestly, for the USD itself to crash would require a combination of extremely improbable events - the USD is a lot less susceptible to rapid movement compared to the stock exchange.