Use post bias to adjust for Y - 16 to improve performance. BUG=388 TESTED=set LIBYUV_DISABLE_ASM=1 out\release\libyuv_unittest --gtest_catch_exceptions=0 --gtest_filter=*I420ToARGB_Opt R=brucedawson@google.com, harryjin@google.com
Use post bias to adjust for Y - 16 to improve performance.
BUG=388
TESTED=set LIBYUV_DISABLE_ASM=1 out\release\libyuv_unittest --gtest_catch_exceptions=0 --gtest_filter=*I420ToARGB_Opt
R=brucedawson@google.com, harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/35609004

git-svn-id: a51711482b6ba0e1d1ab095578a045eeae45ac9a@1213 16f28f9a-4ce2-e073-06de-1de4eb20be90

rig-project/libyuv,archos-sa/libyuv-avp,rig-project/libyuv,archos-sa/libyuv-avp,granalberto/libyuv,granalberto/libyuv,rig-project/libyuv,granalberto/libyuv,granalberto/libyuv,archos-sa/libyuv-avp