Inherit from Arel::Nodes::As instead of Binary

It's done to make it compatible with:

https://github.com/rails/rails/pull/39235/files

config/initializers/postgres_cte_as_materialized.rb

@@ -6,7 +6,7 @@ module Arel
     class Arel::Visitors::PostgreSQL
       def visit_Gitlab_Database_AsWithMaterialized(obj, collector) # rubocop:disable Naming/MethodName
         collector = visit obj.left, collector
-        collector << " AS#{obj.expr} "
+        collector << " AS "
         visit obj.right, collector
       end
     end

lib/gitlab/database/as_with_materialized.rb

@@ -3,19 +3,15 @@
 module Gitlab
   module Database
     # This class is a special Arel node which allows optionally define the `MATERIALIZED` keyword for CTE and Recursive CTE queries.
-    class AsWithMaterialized < Arel::Nodes::Binary
+    class AsWithMaterialized < Arel::Nodes::As
       extend Gitlab::Utils::StrongMemoize
 
-      MATERIALIZED = Arel.sql(' MATERIALIZED')
-      EMPTY_STRING = Arel.sql('')
-      attr_reader :expr
+      MATERIALIZED = 'MATERIALIZED '
 
       def initialize(left, right, materialized: true)
-        @expr = if materialized && self.class.materialized_supported?
-                  MATERIALIZED
-                else
-                  EMPTY_STRING
-                end
+        if materialized && self.class.materialized_supported?
+          right.prepend(MATERIALIZED)
+        end
 
         super(left, right)
       end