[turbofan] Nodes are killed by resetting all their inputs to zero.

TEST=compiler-unittests,mjsunit/numops-fuzz
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/540253003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23725 ce2b1a6d-e550-0410-aec6-3dcde31c8c00