[1.8.x] Fixed #24924 -- Join promotion for multiple Case expressions

Backport of 541f4ea546 from master

django/db/models/query_utils.py

@@ -87,10 +87,8 @@ class Q(tree.Node):
     def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
         # We must promote any new joins to left outer joins so that when Q is
         # used as an expression, rows aren't filtered due to joins.
-        joins_before = query.tables[:]
         clause, joins = query._add_q(self, reuse, allow_joins=allow_joins, split_subq=False)
-        joins_to_promote = [j for j in joins if j not in joins_before]
-        query.promote_joins(joins_to_promote)
+        query.promote_joins(joins)
         return clause
 
     @classmethod