[mlir][sparse] Moved a conditional from the RT library to the generated MLIR.

When generating code to add an element to SparseTensorCOO (e.g., when doing dense=>sparse conversion), we used to check for nonzero values on the runtime side, whereas now we generate MLIR code to do that check.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D110121