Python: Remove flow between ESSA variables

This required a minor change in the type tracker implementation, but
apart from that no other changes appear to be needed. Seems to clean
up the test output quite a bit.

python/ql/src/experimental/dataflow/TypeTracker.qll

@@ -71,7 +71,8 @@ module StepSummary {
 /** Holds if it's reasonable to expect the data flow step from `nodeFrom` to `nodeTo` to preserve types. */
 private predicate typePreservingStep(Node nodeFrom, Node nodeTo) {
   EssaFlow::essaFlowStep(nodeFrom, nodeTo) or
-  jumpStep(nodeFrom, nodeTo)
+  jumpStep(nodeFrom, nodeTo) or
+  nodeFrom = nodeTo.(PostUpdateNode).getPreUpdateNode()
 }
 
 /** Holds if `nodeFrom` steps to `nodeTo` by being passed as a parameter in a call. */

python/ql/src/experimental/dataflow/internal/DataFlowPrivate.qll

@@ -150,22 +150,6 @@ module EssaFlow {
     //   nodeTo is `y` on second line, cfg node
     useToNextUse(nodeFrom.asCfgNode(), nodeTo.asCfgNode())
     or
-    // Refinements
-    exists(EssaEdgeRefinement r |
-      nodeTo.(EssaNode).getVar() = r.getVariable() and
-      nodeFrom.(EssaNode).getVar() = r.getInput()
-    )
-    or
-    exists(EssaNodeRefinement r |
-      nodeTo.(EssaNode).getVar() = r.getVariable() and
-      nodeFrom.(EssaNode).getVar() = r.getInput()
-    )
-    or
-    exists(PhiFunction p |
-      nodeTo.(EssaNode).getVar() = p.getVariable() and
-      nodeFrom.(EssaNode).getVar() = p.getAnInput()
-    )
-    or
     // If expressions
     nodeFrom.asCfgNode() = nodeTo.asCfgNode().(IfExprNode).getAnOperand()
   }