Add Neon implementation of aom_blend_a64_mask

Add Neon implementation of aom_blend_a64_mask as well as the
corresponding test.

Change-Id: I82e3293e0b6b247aea8fddad61ad5fa8ac2db845
diff --git a/test/blend_a64_mask_test.cc b/test/blend_a64_mask_test.cc
index 4c5bd61..5b1330e 100644
--- a/test/blend_a64_mask_test.cc
+++ b/test/blend_a64_mask_test.cc
@@ -259,6 +259,12 @@
                                                      aom_blend_a64_mask_avx2)));
 #endif  // HAVE_AVX2
 
+#if HAVE_NEON
+INSTANTIATE_TEST_SUITE_P(NEON, BlendA64MaskTest8B,
+                         ::testing::Values(TestFuncs(aom_blend_a64_mask_c,
+                                                     aom_blend_a64_mask_neon)));
+#endif  // HAVE_NEON
+
 //////////////////////////////////////////////////////////////////////////////
 // 8 bit _d16 version
 //////////////////////////////////////////////////////////////////////////////