Half float neon port, enable O2 for neon, and denormal test.

source/planar_functions.cc

Index: source/planar_functions.cc
diff --git a/source/planar_functions.cc b/source/planar_functions.cc
index da4c47b831bc8e3b0a79d557adc9dbc0b5491f7c..143ae869dde2cbdf44ade9442accb09a14b83ae9 100644
--- a/source/planar_functions.cc
+++ b/source/planar_functions.cc
@@ -2585,6 +2585,15 @@ int HalfFloatPlane(const uint16* src_y, int src_stride_y,
     }
   }
 #endif
+#if defined(HAS_HALFFLOATROW_NEON)
+  if (TestCpuFlag(kCpuHasNEON)) {
+    HalfFloatRow = HalfFloatRow_Any_NEON;
+    if (IS_ALIGNED(width, 8)) {
+      HalfFloatRow = HalfFloatRow_NEON;
+    }
+  }
+#endif
+
   for (y = 0; y < height; ++y) {
     HalfFloatRow(src_y, dst_y, scale, width);
     src_y += src_stride_y;