add macros to enable/disable code analyst around blocks of code.

Normally these macros should not be used, but if performance
details are wanted for intel code, enable them around the code
and then run via the iaca tool, available on the intel website.

BUG=libyuv:670
TEST=~/iaca-lin64/bin/iaca.sh -64 out/Release/libyuv_unittest
R=wangcheng@google.com

Review-Url: https://codereview.chromium.org/2626193002 .