Add a clz() primitive, backed by a fast compiler intrinsic.

BUG=327095
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/110713002

git-svn-id: svn://svn.chromium.org/blink/trunk@163520 bbb929c8-8fbe-4397-9dbb-9b2b20218538