Simplify feedback vector creation and store in SharedFunctionInfo.

LOG=N
BUG=v8:3212
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/254623002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21085 ce2b1a6d-e550-0410-aec6-3dcde31c8c00