Limit Go explicit CFG child ranking to its consumers

Only function bodies and nodes with epilogue tags need child ranks for explicit sequencing. Restrict the ranking domain instead of ranking children across the entire AST.

Cold ReflectedXss profiles reduce ranked-child rows from 2.09M to 279K on CockroachDB and from 2.69M to 571K on AWS, with unchanged explicit steps and full BQRS results. All 220 Go control-flow/data-flow tests including consistency queries pass. No end-to-end speedup claimed.

Author: owen-mc

go/ql/lib/semmle/go/controlflow/ControlFlowGraphImpl.qll

@@ -1112,9 +1112,15 @@ module CfgImpl {
 
     /**
      * Gets the non-skipped child of `parent` at rank `rnk` (1-based).
-     * This mimics the shared library's getRankedChild but for use in explicit steps.
+     * This mimics the shared library's getRankedChild for explicit sequencing of
+     * function bodies and nodes with epilogues.
      */
     private Ast::AstNode getRankedChild(Ast::AstNode parent, int rnk) {
+      (
+        parent = any(Go::FuncDef fd).getBody()
+        or
+        exists(getEpilogueTag(parent, _))
+      ) and
       result = rank[rnk](Ast::AstNode c, int ix | c = Ast::getChild(parent, ix) | c order by ix)
     }