Fix approximation of CanDeoptimize() and other small store elimination fixes.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/222263002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20468 ce2b1a6d-e550-0410-aec6-3dcde31c8c00