Smarter representation selection for phis.

R=jarin@chromium.org
BUG=

Review URL: https://codereview.chromium.org/641713002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24474 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/master'