[MLIR][Linalg] Avoid padding attribute in `pack` when possible

If we deal with statically known tensors and tiles and a given tile
perfectly divides a given dimension, we can omit the padding attribute.
As a bonus point, we can now run pack and unpack propagation
(currently, we bail out during propagation if we have the padding
attribute).

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D154607