[mlir][sparse] compute allocation size_hint

This adds the hint to a number of tensor allocations in codegens,
shaving off quite some time from e.g. reading in sparse matrices
due to zero-reallocation scheme. Note that we can probably provide
hints on all allocations, and refine the heuristics that use them
for general tensors.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D143309