Improve computation of effects for load elimination.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/152953004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19322 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Omit redundant smi checks in write barriers.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/155843006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19323 ce2b1a6d-e550-0410-aec6-3dcde31c8c00