[OSSACanOwned] Don't walk into reborrow copies.
Because discovery of defs walks into reborrows and borrowed-from
instructions, copies may be seen whose underlying value is a guaranteed
value (namely, a reborrow or a borrowed-from instruction). Such copies
may be used beyond the lifetime end of such guaranteed values, so it's
not allowed to sink copies to their consuming uses. Such
canonicalization is the responsibility of the OSSACanonicalizeGuaranteed
utility.
rdar://139842132
