Improve SSE2 implementation of v64_abs_s8 and v128_abs_s8

Change-Id: I5243432106c2456f1220adb9d8f24ae5e4249748

aom_dsp/simd/v128_intrinsics_x86.h

@@ -124,9 +124,8 @@ SIMD_INLINE v128 v128_abs_s8(v128 a) {
 #if defined(__SSSE3__)
   return _mm_abs_epi8(a);
 #else
-  v128 t = _mm_sub_epi8(_mm_setzero_si128(), a);
-  v128 mask = _mm_cmplt_epi8(t, a);
-  return _mm_or_si128(_mm_andnot_si128(mask, t), _mm_and_si128(mask, a));
+  v128 sign = _mm_cmplt_epi8(a, _mm_setzero_si128());
+  return _mm_xor_si128(sign, _mm_add_epi8(a, sign));
 #endif
 }
 

aom_dsp/simd/v64_intrinsics_x86.h

@@ -142,9 +142,8 @@ SIMD_INLINE v64 v64_abs_s8(v64 a) {
 #if defined(__SSSE3__)
   return _mm_abs_epi8(a);
 #else
-  v64 t = _mm_sub_epi8(_mm_setzero_si128(), a);
-  v64 mask = _mm_cmplt_epi8(t, a);
-  return _mm_or_si128(_mm_andnot_si128(mask, t), _mm_and_si128(mask, a));
+  v64 sign = _mm_cmplt_epi8(a, _mm_setzero_si128());
+  return _mm_xor_si128(sign, _mm_add_epi8(a, sign));
 #endif
 }