Fix CCP assertions

Move the assertions in CCP from the flow-function to after the data-flow has finished. In particular, we check that for each reachable node

• If it is a region node, at least one predecessor is reachable
• If it is a phi node, the control predecessor (region) is reachable and each data input for a reachable predecessor of the region is reachable
• For all other nodes, all uses should be reachable