The check for internalized strings relied on the fact that we had less

The check for internalized strings relied on the fact that we had less
than 64 distinct InstanceTypes. We are hitting that boundary, so this
check needs to be more comprehensive. In fact, two bits need to be
tested: verify that kNotStringTag isn't set, and that kInternalizedTag
is set.

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/17895002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15358 ce2b1a6d-e550-0410-aec6-3dcde31c8c00