[mlir][sparse] Updating `Merger::foreachTensorLoopId` to take `LatPointId`

Since all callsites of `foreachTensorLoopId` would simply look up the `LatPointId` to extract its `BitVector`, it's cleaner to let the `Merger` handle that instead.  This seems to better capture the intent of the `foreachTensorLoopId` method, and improves decoupling (since it removes a place that leaks the implementation detail that we use `BitVector`).

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D146082