Python: Fix bad join in getValue

We were building essentially a CP of all control flow nodes:

```
Tuple counts for Essa::AssignmentDefinition::getValue#dispred#f0820431#ff/2@dd1f67vl after 2m45s:
733365     ~6%     {3} r1 = JOIN Essa::TEssaNodeDefinition#24e22a14#ffff_30#join_rhs WITH Essa::EssaNodeDefinition::getDefiningNode#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Rhs.0
376588     ~0%     {2} r2 = JOIN r1 WITH SsaDefinitions::SsaSource::assignment_definition#9197156e#fff ON FIRST 2 OUTPUT Lhs.2 'this', Rhs.2 'result'
376588     ~0%     {3} r3 = JOIN r2 WITH Essa::TEssaNodeDefinition#24e22a14#ffff_30#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.0 'this', Lhs.1 'result'
6965593033 ~2%     {3} r4 = JOIN r3 WITH project#SsaDefinitions::SsaSource::assignment_definition#9197156e ON FIRST 1 OUTPUT Lhs.1 'this', Rhs.1, Lhs.2 'result'
376588     ~0%     {2} r5 = JOIN r4 WITH Essa::EssaNodeDefinition::getDefiningNode#dispred#f0820431#ff ON FIRST 2 OUTPUT Lhs.0 'this', Lhs.2 'result'
                    return r5
```

We first tried preventing the join on `result`, but this caused the
characteristic predicate to blow up instead. Finally, we figured just
putting the `value` part in a field would be sufficient, and this did
the trick.

Author: tausbn

python/ql/lib/semmle/python/essa/Essa.qll

@@ -498,13 +498,13 @@ private EssaVariable potential_input(EssaNodeRefinement ref) {
 
 /** An assignment to a variable `v = val` */
 class AssignmentDefinition extends EssaNodeDefinition {
+  ControlFlowNode value;
+
   AssignmentDefinition() {
-    SsaSource::assignment_definition(this.getSourceVariable(), this.getDefiningNode(), _)
+    SsaSource::assignment_definition(this.getSourceVariable(), this.getDefiningNode(), value)
   }
 
-  ControlFlowNode getValue() {
-    SsaSource::assignment_definition(this.getSourceVariable(), this.getDefiningNode(), result)
-  }
+  ControlFlowNode getValue() { result = value }
 
   override string getRepresentation() { result = this.getValue().getNode().toString() }