Use links_and_reverse_links_dict in replace_tasks (#824)

In b713497 we added a function to get
both `links_dict` and `reverse_links_dict` in one go, avoiding iterating
over the graph twice to build those. That function also benefits from
being cached. I'm not expecting a big change in performance here but we
might as well.

Author: Eijebong

src/taskgraph/optimize/base.py

@@ -297,8 +297,9 @@ def replace_tasks(
 
     opt_counts = defaultdict(int)
     replaced = set()
-    dependents_of = target_task_graph.graph.reverse_links_dict()
-    dependencies_of = target_task_graph.graph.links_dict()
+    dependencies_of, dependents_of = (
+        target_task_graph.graph.links_and_reverse_links_dict()
+    )
 
     for label in target_task_graph.graph.visit_postorder():
         logger.debug(f"replace_tasks: {label}")