--graphblas-optimize to eliminate useless graphblas.dup calls?

[paul-tqh-nguyen]

While working on #243, I accidentally did wrote some silly code where I was duplicating sparse tensors, not modifying either the duplicate or original tensor, and then using the duplicate and original tensors in different places.

I'm not sure if this is something that should be handled by the user or if this should be automatically fixed by one of our lowering passes.