avcodec/hevcdsp: optimize pixel comparison in sao_edge_filter

GCC 4.9.2 on a Core i5-4200U @ 1.60GHz, Linux x86_64

Before
715487 decicycles in sao_edge_filter_8, 262144 runs, 0 skips

After
672104 decicycles in sao_edge_filter_8, 262144 runs, 0 skips
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/hevcdsp_template.c

@@ -325,7 +325,7 @@ static void FUNC(sao_band_filter_0)(uint8_t *_dst, uint8_t *_src,
     }
 }
 
-#define CMP(a, b) ((a) > (b) ? 1 : ((a) == (b) ? 0 : -1))
+#define CMP(a, b) ((a > b) - (a < b))
 
 static void FUNC(sao_edge_filter)(uint8_t *_dst, uint8_t *_src, ptrdiff_t stride_dst, int16_t *sao_offset_val,
                                   int eo, int width, int height) {