We have to check for uninitialized uses before phi-elimination. Otherwise we
may miss such a use and result in using the hole value instead. This
causes a NULL-dereference or assertion failure.

BUG=96989
TEST=mjsunit/compiler/regress-96989.js
Review URL: http://codereview.chromium.org/7974009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9337 ce2b1a6d-e550-0410-aec6-3dcde31c8c00