The europeans are generally pretty good at keeping the roads open especially if they have a ski resort near the top of a pass the route is the main arterial road into the area. Many routes will be ridable in May however you may find some of the highest routes or minor passes still closed e.g. Stelvio or Col De Bonnette. I have been known to ride up (and down again) some roads which have been closed

and ended in snowdrift, however I am now older and wiser and recognise the French sign for closed road.
Be flexible in your planning and prepared to take the odd detour, and enjoy your holiday.