The result of an Int32 binary operation will be converted to a smi if it fits. However a minus zero check was missing.

BUG=v8:1278
TEST=test/mjsunit/regress/regress-1278.js

R=ager@chromium.org
Review URL: http://codereview.chromium.org/6755009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7399 ce2b1a6d-e550-0410-aec6-3dcde31c8c00