One of the meanings of خَصَّ in Arabic is to have a need, and from which we have the word خَصاصَة: lack, need, poverty.
That's why, when you say : واش/آش خَصَّك - it means what do you need ?
Concerning شحال, it comes probably from آش حال، meaning what's the case/condition/situation of. Therefore it can have two meanings :
1- شحال used in the Maghreb in the meaning of how many/much equivalent to the standard كَم. as well as for [b'shHaal = bi-kam], examples :
- شحال عندك من كتاب ؟ : how many books do you have
- بشحال هذا أو شحال هذا : how much is this ?
2- شحال / اشحال (how is ...) used in the Gulf for example شحالك = how are you ?
إيش، آش used in the meaning of 'what' in many dialects, is derivated from أي شي