LibRegex: Avoid prepending items to vectors

Userland/Libraries/LibRegex/RegexMatcher.cpp

@@ -293,7 +293,7 @@ Optional<bool> Matcher<Parser>::execute(const MatchInput& input, MatchState& sta
     if (recursion_level > c_max_recursion)
         return false;
 
-    Vector<MatchState> fork_low_prio_states;
+    Vector<MatchState, 64> reversed_fork_low_prio_states;
     MatchState fork_high_prio_state;
     Optional<bool> success;
 
@@ -323,7 +323,7 @@ Optional<bool> Matcher<Parser>::execute(const MatchInput& input, MatchState& sta
 
         switch (result) {
         case ExecutionResult::Fork_PrioLow:
-            fork_low_prio_states.prepend(state);
+            reversed_fork_low_prio_states.append(state);
             continue;
         case ExecutionResult::Fork_PrioHigh:
             fork_high_prio_state = state;
@@ -344,8 +344,13 @@ Optional<bool> Matcher<Parser>::execute(const MatchInput& input, MatchState& sta
             return true;
         case ExecutionResult::Failed:
             return false;
-        case ExecutionResult::Failed_ExecuteLowPrioForks:
-            return execute_low_prio_forks(input, state, output, fork_low_prio_states, recursion_level + 1);
+        case ExecutionResult::Failed_ExecuteLowPrioForks: {
+            Vector<MatchState> fork_low_prio_states;
+            fork_low_prio_states.ensure_capacity(reversed_fork_low_prio_states.size());
+            for (ssize_t i = reversed_fork_low_prio_states.size() - 1; i >= 0; i--)
+                fork_low_prio_states.unchecked_append(move(reversed_fork_low_prio_states[i]));
+            return execute_low_prio_forks(input, state, output, move(fork_low_prio_states), recursion_level + 1);
+        }
         }
     }