C++: data flow AlwaysTrueUponEntryLoop perf fix

The predicate `AlwaysTrueUponEntryLoop.getARelevantVariable` was very
sensitive to join ordering, and with the 1.19 QL engine it got an
unfortunate join order that made it explode on certain snapshots. With
this change, it goes from taking minutes to taking less than a second on
a libretro-uae snapshot.

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

@@ -292,10 +292,7 @@ module FlowVar_internal {
      * Gets a variable that is assigned in this loop and read outside the loop.
      */
     private Variable getARelevantVariable() {
-      exists(BasicBlock bbAssign |
-        assignmentLikeOperation(bbAssign.getANode(), result, _) and
-        this.bbInLoop(bbAssign)
-      ) and
+      result = this.getAVariableAssignedInLoop() and
       exists(VariableAccess va |
         va.getTarget() = result and
         readAccess(va) and
@@ -303,6 +300,15 @@ module FlowVar_internal {
       )
     }
 
+    /** Gets a variable that is assigned in this loop. */
+    pragma[noinline]
+    private Variable getAVariableAssignedInLoop() {
+      exists(BasicBlock bbAssign |
+        assignmentLikeOperation(bbAssign.getANode(), result, _) and
+        this.bbInLoop(bbAssign)
+      )
+    }
+
     private predicate bbInLoopCondition(BasicBlock bb) {
       getCFNParent*(bb.getANode()) = this.(Loop).getCondition()
     }