Introduce `tensor.pack` and `tensor.unpack` operations

Pack and Unpack return new tensors within which the individual elements
are reshuffled according to the packing specification. This has the
consequence of modifying the canonical order in which a given operator
(i.e., Matmul) accesses the individual elements. After bufferization,
this typically translates to increased access locality and cache
behavior improvement, e.g., eliminating cache line splitting.

Co-authored-by: Mahesh Ravishankar <ravishankarm@google.com>
Co-authored-by: Han-Chung Wang <hanchung@google.com>

RFC: https://discourse.llvm.org/t/rfc-tensor-pack-and-tensor-unpack/66408/1

Reviewed By: nicolasvasilache, rengolin, hanchung

Differential Revision: https://reviews.llvm.org/D138119