I found this bug by inspection.
This is an important bug to fix since this pass runs at -Onone and the bug
results in the compiler hitting an unreachable.
The way the unreachable is triggered is that when we detect that we are going to
promote a box, if we see a (struct_element_addr (project_box box)), we don't map
the struct_element_addr to a cloned value. If we have a load, this is not an
issue, since we are mapping the load to the struct_extract. But if we have /any/
other non-load users of the struct_element_addr, the cloner will attempt to look
up the struct_element_addr and will be unable to find it, hitting an
unreachable.
rdar://32776202