Basic compiler support for descendant-or-self

The generated code isn't entirely correct which considering the tests do
pass means the tests need to be fixed too.

lib/oga/xpath/compiler.rb

@@ -214,6 +214,20 @@ module Oga
         end
       end
 
+      # @param [AST::Node] ast
+      # @param [Oga::Ruby::Node] input
+      # @return [Oga::Ruby::Node]
+      def on_axis_descendant_or_self(ast, input, &block)
+        descendant = node_literal
+        self_test  = process(ast, input, &block).if_true { yield input }
+
+        descendants_test = input.each_node.add_block(descendant) do
+          process(ast, descendant, &block).if_true { yield descendant }
+        end
+
+        self_test.followed_by(descendants_test)
+      end
+
       # @param [AST::Node] ast
       # @param [Oga::Ruby::Node] input
       # @return [Oga::Ruby::Node]