Add ARGBExtractAlpha_AVX2 function

source/planar_functions.cc

Index: source/planar_functions.cc
diff --git a/source/planar_functions.cc b/source/planar_functions.cc
index 45bae1ffd1dfbaeb198bf6fdc8c0e3ba3f3b98f8..3838759be1ebdb507b84c4719c9be617eb5f3138 100644
--- a/source/planar_functions.cc
+++ b/source/planar_functions.cc
@@ -2702,6 +2702,12 @@ int ARGBExtractAlpha(const uint8* src_argb, int src_stride,
                                                : ARGBExtractAlphaRow_Any_SSE2;
   }
 #endif
+#if defined(HAS_ARGBEXTRACTALPHAROW_AVX2)
+  if (TestCpuFlag(kCpuHasAVX2)) {
+    ARGBExtractAlphaRow = IS_ALIGNED(width, 32) ? ARGBExtractAlphaRow_AVX2
+                                                : ARGBExtractAlphaRow_Any_AVX2;
+  }
+#endif
 #if defined(HAS_ARGBEXTRACTALPHAROW_NEON)
   if (TestCpuFlag(kCpuHasNEON)) {
     ARGBExtractAlphaRow = IS_ALIGNED(width, 16) ? ARGBExtractAlphaRow_NEON