Fix horizontal/horizontal_up 8x8l intra prediction x86/simd functions.
The original functions did not work correctly for edge pixels, e.g.
when CODEC_FLAG_EMU_EDGE is set, leading to corrupt output in e.g. VLC.
Based on a patch by Daniel Kang <daniel d kang gmail com>.

Signed-off-by: Ronald S. Bultje <rsbultje gmail com>
