Implement graph trimming in ControlReducer.

Trimming the graph consists of breaking links from nodes that are not reachable from end to nodes that are reachable from end. Such dead nodes show up in the use lists of the live nodes and though mostly harmless, just clutter up the graph. They also can limit instruction selection opportunities, so it is good to get rid of them.

This CL is one half of the ControlReducer functionality, the other half
being branch folding.

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/661923002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24694 ce2b1a6d-e550-0410-aec6-3dcde31c8c00