[mlir][linalg] Improve the padding packing loop computation.

The revision updates the packing loop search in hoist padding. Instead of considering all loops in the backward slice, we now compute a separate backward slice containing the index computations only. This modification ensures we do not add packing loops that are not used to index the packed buffer due to spurious dependencies. One instance where such spurious dependencies can appear is the extract slice operation introduced between the tile loops of a double tiling.

Depends On D112412

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D112713