Add JSGraph::GetCachedNodes and NodeCache::GetCachedNodes. These routines are necessary in the dead code elimination phase to trim away uses from unreachable nodes.

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/656103002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24632 ce2b1a6d-e550-0410-aec6-3dcde31c8c00