C++: Optimize SubBasicBlock::getNode(int)

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

@@ -97,16 +97,24 @@ class SubBasicBlock extends ControlFlowNodeBase {
    * start from 0, and the node at position 0 always exists and compares equal
    * to `this`.
    */
+  pragma[nomagic]
   ControlFlowNode getNode(int pos) {
-    exists(BasicBlock bb | bb = this.getBasicBlock() |
-      exists(int thisPos | this = bb.getNode(thisPos) |
-        result = bb.getNode(thisPos + pos) and
-        pos >= 0 and
-        pos < this.getNumberOfNodes()
+    exists(BasicBlock bb |
+      exists(int outerPos |
+        result = bb.getNode(outerPos) and
+        pos = outerPosToInnerPos(bb, outerPos)
       )
     )
   }
 
+  pragma[nomagic]
+  private int outerPosToInnerPos(BasicBlock bb, int posInBB) {
+    exists(int thisPosInBB | this = bb.getNode(thisPosInBB) |
+      posInBB = result + thisPosInBB and
+      result = [ 0 .. this.getNumberOfNodes() - 1 ]
+    )
+  }
+
   /** Gets a control-flow node in this `SubBasicBlock`. */
   ControlFlowNode getANode() {
     result = this.getNode(_)