doing constant folding, handle only the cases that actually arise.
This avoids some complicated logic, some extra comparisons, and fixes
issue 247:

http://code.google.com/p/v8/issues/detail?id=247

Review URL: http://codereview.chromium.org/40023

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1410 ce2b1a6d-e550-0410-aec6-3dcde31c8c00