[mlir][sparse] start using size_hint provided in allocation op

Even though we introduced the size_hint, we never used it.
This is a very first step, using the hint during the codegen path.
Note that we can refine the heuristics. Also, we need to start
adding the hint on all allocation generated for reading tensors,
converting tensors, etc.

Reviewed By: Peiming, bixia

Differential Revision: https://reviews.llvm.org/D143292