[mlir][linalg] Fix rank-reduced cases for extract/insert slice in DropUnitDims (#74723)

Inferring the reshape reassociation indices for extract/insert slice ops
based on the read sizes of the original slicing op will generate an
invalid expand/collapse shape op for already rank-reduced cases. Instead
just infer from the shape of the slice.

Ported from Differential Revision: https://reviews.llvm.org/D147488