Properly handle undefined conversion to zero in Crankshaft.

R=yangguo@chromium.org
BUG=none
TEST=regress-1563.js

Review URL: http://codereview.chromium.org/7461028

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8723 ce2b1a6d-e550-0410-aec6-3dcde31c8c00