[mlir][sparse] change memref argument to proper SSA components

The indices for insert/compress were previously provided as
a memref<?xindex> with proper rank, since that matched the
argument for the runtime support libary better. However, with
proper codegen coming, providing the indices as SSA values
is much cleaner. This also brings the sparse_tensor.insert
closer to unification with tensor.insert, planned in the
longer run.

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D134404