Fix handling of potential string additions in hydrogen.

R=titzer@chromium.org
TEST=mjsunit/regress/regress-3476
BUG=v8:3476
LOG=N

Review URL: https://codereview.chromium.org/423083004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22677 ce2b1a6d-e550-0410-aec6-3dcde31c8c00