It's measured faster when:
a) clamp never happens;
b) clamp random happens ([-128,384], pseudo random).

Review URL: https://chromiumcodereview.appspot.com/11190049
Patch from Zheng Liu <zheng.z.liu@intel.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12777 ce2b1a6d-e550-0410-aec6-3dcde31c8c00