Fix ElementsKind handling of prototypes in Array.concat

Double elements, typed elements, and sloppy arguments elements were all erroneously marked UNREACHABLE.

BUG=chromium:412203
LOG=n
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/560463002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23863 ce2b1a6d-e550-0410-aec6-3dcde31c8c00