[Mem2Reg] NFC: Entype'd LiveValues ownership.

Previously, LiveValues consisted always of three values: the value which
was stored, the borrow, and the copy.  For store_borrows, there never
was a copy.  Treating the two different scenarios as if they were the
same was confusing already.  It was only get when we switch to
representing owned lexical lifetimes with move_values.

Author: nate-chandler