[mlir][sparse] refactor handling of merger leafs and ops

Using "default:" in the switch statemements that handle all our
merger ops has become a bit cumbersome since it is easy to overlook
parts of the code that need to handle ops specifically. By enforcing
full switch statements without "default:", we get a compiler warning
when cases are overlooked.

Reviewed By: wrengr

Differential Revision: https://reviews.llvm.org/D127263