swift234

Don't assert on alloc_stack with only debug_value_addr uses.

This came up with other changes I have to modify the optimizer
pipeline. We shouldn't assert if we have an alloc_stack/dealloc_stack
where the only other use of the alloc_stack is a debug_value_addr.

It's easy to avoid this by removing allocations that don't have real
uses prior to attempting to handle the ones that do have real uses (as
opposed to the other way around).