This new OSSA invariant simplifies many optimizations because they don't have to take care of the corner case of incomplete lifetimes in dead-end blocks.
The implementation basically consists of these changes:
* add the lifetime completion utility
* add a flag in SILFunction which tells optimization that they need to run the lifetime completion utility
* let all optimizations complete lifetimes if necessary
* enable the ownership verifier to check complete lifetimes
When its operand has coroutine kind `yield_once_2`, a `begin_apply`
instruction produces an additional value representing the storage
allocated by the callee. This storage must be deallocated by a
`dealloc_stack` on every path out of the function. Like any other stack
allocation, it must obey stack discipline.
The areUsesWithinBoundary/areUsesOutsideBoundary methods take the
dead-ends because the region that they're checking for containment
within contains more than just (is a (non-strict) superset of) the
pruned live region. On the other hand, the region also contains _less_
than (is a (non-strict) subset of) the available region. Instructions
are in this extended region only if they're in a dead-end block which is
forwards reachable from the non-lifetime-ending liveness boundary.
