Skip dead blocks/instructions in store elimination.

Also improve tracing.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/297933002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21463 ce2b1a6d-e550-0410-aec6-3dcde31c8c00