It's not possible to configure keyboard shortcuts to tile windows to corners using the xfce 4.12 release. To fix this, the following lines should be added to libxfce4ui/libxfce4kbd-private/xfce-shortcuts-xfwm4.c:

  { N_("Tile window to the bottom-left"), "tile_down_left_key" },
  { N_("Tile window to the bottom-right"), "tile_down_right_key" },
  { N_("Tile window to the top-left"), "tile_up_left_key" },
  { N_("Tile window to the top-right"), "tile_up_right_key" },