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

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 01b023ee62da8219f12c574ea4548012bed04f7c..04bf80ffff6c64ecf1d7c4a718cfe395d36db08b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -122,6 +122,10 @@ static_library("libyuv") {
     # Enable optimize for speed (-O2) over size (-Os).
     configs += [ "//build/config/compiler:optimize_max" ]
   }
+
+  # To enable AVX2 or other cpu optimization, pass flag here
+  #  cflags = [ "-mavx2" ]
+
 }
 
 if (libyuv_use_neon) {
@@ -140,6 +144,14 @@ if (libyuv_use_neon) {
 
     public_configs = [ ":libyuv_config" ]
 
+    # Always enable optimization for Release and NaCl builds (to workaround
+    # crbug.com/538243).
+    if (!is_debug) {
+      configs -= [ "//build/config/compiler:default_optimization" ]
+      # Enable optimize for speed (-O2) over size (-Os).
+      configs += [ "//build/config/compiler:optimize_max" ]
+    }
+
     if (current_cpu != "arm64") {
       configs -= [ "//build/config/compiler:compiler_arm_fpu" ]
       cflags = [ "-mfpu=neon" ]