[Attributor][FIX] Avoid H2S on GPUs if the pointer can be shared

[Attributor][FIX] Avoid H2S on GPUs if the pointer can be shared

If the stack is not accessible by other threads, e.g., on a GPU, we need
to ensure heap-2-stack will not create a stack version of a pointer that
might be passed to another thread. Since passing through memory is by
default transparent, we need to register a callback and inspect stores
we might look through explicitly.