[mlir][sparse] Factoring out `finalizeSegment` and (generic) `appendIndex`

This change introduces two new methods: `finalizeSegment` and `appendIndex`; and removes three old methods: `endDim`, `appendCurrentPointer`, `appendIndex`.  The two new methods better encapsulate their algorithms, thus allowing to remove repetitious code in several other places.

Depends On D122435

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D122625