[mlir][linalg] Enhance FoldInsertPadIntoFill to support op chain

If we have a chain of `tensor.insert_slice` ops inserting some
`tensor.pad` op into a `linalg.fill` and ranges do not overlap,
we can also elide the `tensor.pad` later.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D120446