Can s7 Wall charger be used on s8?
Samsung says 


  ....You can charge your phone using any Samsung-approved wall charger, but charging speeds may vary depending on the charger....


Further this reddit claims that S7 and S8 use the same charger

I am not sure what is the difference in charging speeds

In the absence of specs , it is not possible to say for sure, but I wouldn't hesitate since both chargers are Samsung and apparently similar as pointed in the reddit.