Python: Fix another bad TC.

This one is a bit awkward, since the previous version was supposed to
improve indexing. Unfortunately this is vastly outweighed by the slow
convergence of the TC. Right now we pay the cost of inverting the
`hasFlowSource` relation, but this is still cheaper.

Author: tausbn

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -530,15 +530,12 @@ private module Cached {
    * The slightly backwards parametering ordering is to force correct indexing.
    */
   cached
-  predicate hasLocalSource(Node sink, Node source) {
-    // Declaring `source` to be a `SourceNode` currently causes a redundant check in the
-    // recursive case, so instead we check it explicitly here.
-    source = sink and
-    source instanceof LocalSourceNode
+  predicate hasLocalSource(Node sink, LocalSourceNode source) {
+    source = sink
     or
-    exists(Node mid |
-      hasLocalSource(mid, source) and
-      simpleLocalFlowStep(mid, sink)
+    exists(Node second |
+      simpleLocalFlowStep(source, second) and
+      simpleLocalFlowStep*(second, sink)
     )
   }