Fix DeepCompile last use for unconsumed waits (#8254)
With this PR, DeepCompile treats an unconsumed `wait_allgather` alias as
its own last use to prevent `KeyError`, while preserving downstream
propagation for consumed aliases.
Add synthetic FX regression coverage for both orphan-only and
later-real-use cases.
Signed-off-by: Masahiro Tanaka <mtanaka@anyscale.com>
