We should only mark a string as an array index if we can store the entire value
of the number in the hash field. We sometimes failed to reject larger numbers.

Fixes http://code.google.com/p/v8/issues/detail?id=728

Review URL: http://codereview.chromium.org/2452007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4782 ce2b1a6d-e550-0410-aec6-3dcde31c8c00