make isStartState public in ReDoSUtil

python/ql/lib/semmle/python/security/performance/ReDoSUtil.qll

@@ -803,29 +803,26 @@ InputSymbol getAnInputSymbolMatching(string char) {
   result = Any()
 }
 
+/**
+ * Holds if `state` is a start state.
+ */
+predicate isStartState(State state) {
+  state = mkMatch(any(RegExpRoot r))
+  or
+  exists(RegExpCaret car | state = after(car))
+}
+
 /**
  * Predicates for constructing a prefix string that leads to a given state.
  */
 private module PrefixConstruction {
-  /**
-   * Holds if `state` starts the string matched by the regular expression.
-   */
-  private predicate isStartState(State state) {
-    state instanceof StateInPumpableRegexp and
-    (
-      state = Match(any(RegExpRoot r), _)
-      or
-      exists(RegExpCaret car | state = after(car))
-    )
-  }
-
   /**
    * Holds if `state` is the textually last start state for the regular expression.
    */
   private predicate lastStartState(State state) {
     exists(RegExpRoot root |
       state =
-        max(State s, Location l |
+        max(StateInPumpableRegexp s, Location l |
           isStartState(s) and getRoot(s.getRepr()) = root and l = s.getRepr().getLocation()
         |
           s