Otherwise dead phi elimination can actually remove some of the implicitly used phis.

BUG=1257
TEST=test/mjsunit/regress/regress-1257.js

Review URL: http://codereview.chromium.org/6672066

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7221 ce2b1a6d-e550-0410-aec6-3dcde31c8c00