Fix Node::ReplaceUses to handle unused nodes.

R=bmeurer@chromium.org
TEST=cctest/test-node/NodeUseIteratorReplaceNoUses

Review URL: https://codereview.chromium.org/521643002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23519 ce2b1a6d-e550-0410-aec6-3dcde31c8c00