[mlir][TransformDialect] Simplify the lowering of pack/unpack when these are just pad/unpad

This patch recognizes when tensor.pack/unpack operations are simple
tensor.pad/unpad (a.k.a. tensor.extract_slice) and lowers them in a simpler
sequence of instruction.

For pack, instead of doing:
```
pad
expand_shape
transpose
```
we do
```
pad
insert_slice
```

For unpack, instead of doing:
```
transpose
collapse_shape
extract_slice
```
we do
```
extract_slice
```

Note: returning nullptr for the transform dialect is fine. The related
handles are just ignored by the following transformation.

Differential Revision: https://reviews.llvm.org/D148159