Revamp descendant-or-self compiler specs

lib/oga/xpath/compiler.rb

@@ -204,15 +204,17 @@ module Oga
       # @param [Oga::Ruby::Node] input
       # @return [Oga::Ruby::Node]
       def on_axis_descendant_or_self(ast, input)
-        node = node_literal
+        node = unique_literal(:descendant)
 
-        process(ast, input)
-          .if_true { yield input }
-          .followed_by do
-            input.each_node.add_block(node) do
-              process(ast, node).if_true { yield node }
+        document_or_node(input).if_true do
+          process(ast, input)
+            .if_true { yield input }
+            .followed_by do
+              input.each_node.add_block(node) do
+                process(ast, node).if_true { yield node }
+              end
             end
-          end
+        end
       end
 
       # @param [AST::Node] ast