[turbofan] More const-correctness changes.

Also get rid of the DeleteNode and ChangeOperator methods in Graph.

TEST=compiler-unittests,cctest,mjsunit
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/540863002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23684 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix invalid reinterpret_cast.

TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/544583002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23685 ce2b1a6d-e550-0410-aec6-3dcde31c8c00