parse.y: expand is_asgn_or_id

* parse.y (new_op_assign_gen): expand is_asgn_or_id(), which call
  id_type() function multiple times now.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46754 b2dd03c8-39d4-4d8f-98ff-823fe69b080e