don't use Loop._invariants in Loop._node

The `Loop._invariants` are quite expensive to build. This patch aims to reduce
the usage of `Loop._invariants` by reimplementing `Loop._node` without
`Loop._invariants`.

nutils/evaluable.py

@@ -4521,8 +4521,16 @@ def evalf_loop_body_withtimes(self, times, output, body_arg):
     def _node(self, cache, subgraph, times, unique_loop_ids):
         if (cached := cache.get(self)) is not None:
             return cached
-        for arg in itertools.chain(self._invariants, (self.init_arg,)):
-            arg._node(cache, subgraph, times, unique_loop_ids)
+
+        # Populate the `cache` with objects that do not depend on `self.index`.
+        stack = [self.init_arg, self.body_arg]
+        while stack:
+            func = stack.pop()
+            if self.index in func.arguments:
+                stack.extend(func._Evaluable__args)
+            else:
+                func._node(cache, subgraph, times, unique_loop_ids)
+
         if unique_loop_ids:
             loopcache = cache
             loopgraph = cache.setdefault(('subgraph', self.loop_id), Subgraph('Loop', subgraph))