[mlir][sparse] Factoring out type-based function-name suffixes

Depends On D115010

This changes a couple of places that used to `return failure();` to now use `llvm_unreachable()` instead. However, `Transforms/Sparsification.cpp` should be doing the necessary type checks to ensure that those cases are in fact unreachable.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D115012