This fixes a corner case introduced by escape analysis where phis are
introduced in OSR loop entry blocks that don't have a merge index and
hence cannot contain OSR values.

R=titzer@chromium.org
TEST=mjsunit/compiler/escape-analysis

Review URL: https://codereview.chromium.org/23503025

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16484 ce2b1a6d-e550-0410-aec6-3dcde31c8c00