vp8: [loongson] optimize loopfilter v2.
vp8: [loongson] optimize loopfilter v2.

Optimize function vp8_mbloop_filter_vertical_edge_mmi and
function vp8_mbloop_filter_horizontal_edge_mmi.
Make full use of memory loading delay slot and reduce unnecessary
instructions.

Change-Id: I61da2c3a44c06044225461f46bf487d83cba6c16

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx