This adds generic support for ExternalReferences in Hydrogen (and Lithium), as required for AddIncrementCounter.

R=danno@chromium.org, titzer@chromium.org

Review URL: https://codereview.chromium.org/19562003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15936 ce2b1a6d-e550-0410-aec6-3dcde31c8c00