[mlir][sparse] Updating the `Merger::{exp,lat,set}` methods to return const

This helps the `Merger` maintain invariants, as well as clarifying the immutability of the underlying objects (with the one exception of `TensorExp::val`).

Depends On: D146559

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D146083