The question is a total waste of time.   If quantum computing were to become available and 1 system suddenly had the computing power of all the computers ever built, bitcoin is the least of our troubles.   Bitcoin is built on top of asymmetric cryptography.  Well, so is everything else,   all of banking and all of modern digital commerce would fall under attack.   

If the concept of quantum computing becomes reality,   there will be a million things more important then bitcoin which may be destroyed.