cache the BasicBlock charpred

javascript/ql/src/semmle/javascript/BasicBlocks.qll

@@ -119,7 +119,8 @@ private predicate bbIPostDominates(BasicBlock dom, BasicBlock bb) =
  * At the database level, a basic block is represented by its first control flow node.
  */
 class BasicBlock extends @cfg_node, NodeInStmtContainer {
-  BasicBlock() { startsBB(this) }
+  cached
+  BasicBlock() { Stages::BasicBlocks::ref() and startsBB(this) }
 
   /** Gets a basic block succeeding this one. */
   BasicBlock getASuccessor() { succBB(this, result) }