[Attributor] Make sure Heap2Stack works properly on a GPU target
If the target stack is not accessible between different running
"threads" we have to make sure not to create allocas for mallocs
that might be used by multiple "threads". The "use check" is
sufficient to prevent this but if we apply the "free check" we have
to make sure the pointer is not communicated to others before
the free is reached.
Differential Revision: https://reviews.llvm.org/D98608