C++: Fix join order in reachesWithoutAssignment

The negation in this predicate did not get pulled into an
`#antijoin_rhs` predicate but got materialized as part of each
iteration, which meant that the temporary `ControlFlowNode` column did
not get projected away. The tuple counts looked like this on
kamailio/kamailio (iteration 20):

    5724      ~13%      {3} r9 = JOIN r8 WITH BasicBlocks::Cached::bb_successor_cached#ff@staged_ext AS R ON FIRST 2 OUTPUT r8.<2>, r8.<3>, r8.<1>
    5724      ~12%      {3} r10 = JOIN r8 WITH BasicBlocks::Cached::bb_successor_cached#ff@staged_ext AS R ON FIRST 2 OUTPUT r8.<3>, r8.<2>, r8.<1>
    124717061 ~11%      {4} r11 = JOIN r10 WITH project#FlowVar::FlowVar_internal::assignmentLikeOperation#ffff_10#join_rhs AS R ON FIRST 1 OUTPUT R.<1>, r10.<2>, r10.<1>, r10.<0>
    66        ~0%       {3} r12 = JOIN r11 WITH project#BasicBlocks::Cached::basic_block_member AS R ON FIRST 2 OUTPUT r11.<2>, r11.<3>, r11.<1>
    66                  {3} r13 = MATERIALIZE r12 AS antijoin_rhs
    5658      ~14%      {3} r14 = r9 AND NOT r13(r9.<0>, r9.<1>, r9.<2>)

After manually pulling out the join inside the negation, the time per
iteration drops from ~30 to <1s. The pipeline above is replaced with

    892394  ~0%      {4} r6 = r5 AND NOT FlowVar::FlowVar_internal::assignsToVar#fb AS R(r5.<3>, r5.<2>)
    892394  ~0%      {4} r7 = SCAN r6 OUTPUT r6.<1>, r6.<3>, r6.<0>, r6.<2>
    5658    ~11%     {3} r8 = JOIN r7 WITH BasicBlocks::Cached::bb_successor_cached#ff@staged_ext AS R ON FIRST 2 OUTPUT r7.<2>, r7.<1>, r7.<3>

Author: jbj

cpp/ql/src/semmle/code/cpp/dataflow/internal/FlowVar.qll

@@ -427,7 +427,7 @@ module FlowVar_internal {
     /**
      * Gets a variable that is assigned in this loop and read outside the loop.
      */
-    private Variable getARelevantVariable() {
+    Variable getARelevantVariable() {
       result = this.getAVariableAssignedInLoop() and
       exists(VariableAccess va |
         va.getTarget() = result and
@@ -472,10 +472,16 @@ module FlowVar_internal {
         reachesWithoutAssignment(bb.getAPredecessor(), v) and
         this.bbInLoop(bb)
       ) and
-      not assignmentLikeOperation(bb.getANode(), v, _, _)
+      not assignsToVar(bb, v)
     }
   }
 
+  pragma[noinline]
+  private predicate assignsToVar(BasicBlock bb, Variable v) {
+    assignmentLikeOperation(bb.getANode(), v, _, _) and
+    exists(AlwaysTrueUponEntryLoop loop | v = loop.getARelevantVariable())
+  }
+
   /**
    * Holds if `loop` always assigns to `v` before leaving through an edge
    * from `bbInside` in its condition to `bbOutside` outside the loop. Also,