[Transform] Support more case for the transform pad operation

Don't choke on `outs` arguments that are not produced by `tensor.empty` or
`tensor.extract_slice`.

When the `outs` argument has a static shape we have all the necessary
information to proceed with the padding.

This makes the `transform.structured.pad` a little bit more resilient.

Differential Revision: https://reviews.llvm.org/D150112