Allow constants to be deleted by reinserting them into the graph as needed.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/31943002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17299 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
HEnvironmentMarker factory added

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/32513002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17300 ce2b1a6d-e550-0410-aec6-3dcde31c8c00