[MLIR][Tensor] Add canonicalization patterns for `tensor.pack`

- Fold an unpack(pack(x)) to x.

- Rewrite a `tensor.pack` to an `tensor.expand_shape` if only one
  dimension is packed.

Reviewed By: tyb0807, hanchung, mravishankar

Differential Revision: https://reviews.llvm.org/D141123