Move extensibility check to the top of Object.isFrozen/Object.isSealed

This speeds up isFrozen/isSealed checks on "normal" objects without
slowing down checks on frozen/sealed objects.

Though this ordering is not what ES5 specifies, the difference is not
observable (especially since the code bails out if the passed-in object
is a proxy).

Review URL: https://codereview.chromium.org/12340008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13726 ce2b1a6d-e550-0410-aec6-3dcde31c8c00