[OSSA] Fix borrowCopyOverGuaranteedUsers dead-ends.
The utility was computing the boundary of the `copy_value` with the
`end_borrow`s as users. But there may not be any `end_borrow`s thanks
to dead-ends. Instead, consider both guaranteed users and any
`end_borrow`s.
Fixes a miscompile in CSE.
rdar://158353230
