Cfg: Add consistency check for relevant child indices.

Author: aschackmull

shared/controlflow/codeql/controlflow/ControlFlowGraph.qll

@@ -2090,6 +2090,12 @@ module Make0<LocationSig Location, AstSig<Location> Ast> {
         module Consistency {
           /** Holds if the consistency query `query` has `results` results. */
           query predicate consistencyOverview(string query, int results) {
+            query = "siblingsWithSameIndexInDefaultCfg" and
+            results =
+              strictcount(AstNode parent, AstNode child1, AstNode child2, int i |
+                siblingsWithSameIndexInDefaultCfg(parent, child1, child2, i)
+              )
+            or
             query = "deadEnd" and results = strictcount(ControlFlowNode node | deadEnd(node))
             or
             query = "nonUniqueEnclosingCallable" and
@@ -2135,6 +2141,20 @@ module Make0<LocationSig Location, AstSig<Location> Ast> {
             results = strictcount(ControlFlowNode node, SuccessorType t | selfLoop(node, t))
           }
 
+          /**
+           * Holds if `parent` uses default left-to-right control flow and has
+           * two different children `child1` and `child2` at the same index
+           * `i`.
+           */
+          query predicate siblingsWithSameIndexInDefaultCfg(
+            AstNode parent, AstNode child1, AstNode child2, int i
+          ) {
+            defaultCfg(parent) and
+            getChild(parent, i) = child1 and
+            getChild(parent, i) = child2 and
+            child1 != child2
+          }
+
           /**
            * Holds if `node` is lacking a successor.
            *