Python: Filter _before_ the cartesian product

It's always a sad thing to see a good plan go wrong:

86860032 ~0%      {4} r26 = JOIN r19 WITH DataFlowPublic::TupleElementContent#class#ff CARTESIAN PRODUCT OUTPUT Lhs.0 'nodeFrom', Lhs.1 'nodeTo', Rhs.0, Rhs.1
129256   ~3%      {4} r27 = SELECT r26 ON In.3 <= 7
129256   ~0%      {3} r28 = SCAN r27 OUTPUT In.0 'nodeFrom', In.2 'c', In.1 'nodeTo'

Happily, now it looks like this:

129256  ~0%      {3} r20 = JOIN r19 WITH DataFlowPrivate::small_tuple#f CARTESIAN PRODUCT OUTPUT Lhs.0 'nodeFrom', Rhs.0, Lhs.1 'nodeTo'

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

@@ -1517,10 +1517,13 @@ predicate forReadStep(CfgNode nodeFrom, Content c, Node nodeTo) {
     or
     c instanceof SetElementContent
     or
-    c.(TupleElementContent).getIndex() <= 7
+    c = small_tuple()
   )
 }
 
+pragma[noinline]
+TupleElementContent small_tuple() { result.getIndex() <= 7 }
+
 /**
  * Holds if `nodeTo` is a read of an attribute (corresponding to `c`) of the object in `nodeFrom`.
  *