Added two explicit types to avoid undesired behaviour of Rascal typechecker

[PaulKlint]

Two trivial changes (adding an explicit FlowEdge type to both es) to make the Rascal typechecker -- and hopefully us ;-) -- happier:

FlowEdge getBlockEdge(CFGNode n1, CFGNode n2) {
		n1Exit = last(n1.nodes);
		n2Entry = head(n2.nodes);
		edges = { e | FlowEdge e <- g.edges, n1Exit.lab == e.from, n2Entry.lab == e.to };
		if (size(edges) > 1) {
			for (FlowEdge e <- edges)
				println("In <g.item>, found flow edge <e>"); 
			throw "We should not have multiple edges between the same nodes";
		}
		e = getOneFrom(edges);
		return e[from=n1.lab][to=n2.lab];
	}

[mahills]

I'm fine with that. I'm curious if changing the declaration of edges to set[FlowEdge] edges would have the same impact (there's no need to do it, I'm just legitimately curious).