[mlir][Linalg] NFC: Clean up for 0-D abstraction.

Summary:
After D75831 has been landed, both the generic op and indexed_generic op can
handle 0-D edge case. In the previous patch, only generic op has been updated.
This patch updates the lowering to loops for indexed_generic op. Since they are
almost the sanme, the patch also refactors the common part.

Differential Revision: https://reviews.llvm.org/D76413