Don't include arg -> param edges in PathGraph::edges whose arg is not reachable

This avoids lots of missing-node warnings from `codeql bqrs interpret` as it discards the nodes that occur in the `edges` relation but not `nodes`. The problem arises because subpaths introduced two variants of `reach`, one of which is more restrictive than simply `reach(succ) and succ = pred.getASuccessor()`, so it no longer suffices to just check that the successor is reachable.

Author: smowton

java/ql/lib/semmle/code/java/dataflow/internal/DataFlowImpl.qll

@@ -3742,7 +3742,7 @@ private predicate pathSuccPlus(PathNode n1, PathNode n2) = fastTC(pathSucc/2)(n1
  */
 module PathGraph {
   /** Holds if `(a,b)` is an edge in the graph of data flow path explanations. */
-  query predicate edges(PathNode a, PathNode b) { a.getASuccessor() = b and reach(b) }
+  query predicate edges(PathNode a, PathNode b) { a.getASuccessor() = b and reach(a) and reach(b) }
 
   /** Holds if `n` is a node in the graph of data flow path explanations. */
   query predicate nodes(PathNode n, string key, string val) {