Handle specially the cases when the argument is a typed array,
in particular of the same type as the one we create.

Allocate backing store uninitialized in cases when we can guarantee
complete initialization.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/21369002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15998 ce2b1a6d-e550-0410-aec6-3dcde31c8c00