Upstream pack and unpack

[rengolin]

We only have two ops in linalgx: pack and unpack. These ops have been in IREE for a while, but we need to make sure they make it to linalg in MLIR, or we'll be stuck with this forever.

We need to create a few examples on pure linalg code and just push those ops into linalg itself as an upstream patch on Phabricator. This will give more people, outside of IREE, visibility on what we're trying to do and help us merge this soon.

I'm hoping @nicolasvasilache can help us here. We're not aiming at perfect, we're just aiming are minimally acceptable, so that we can iterate upstream rather than carrying this with us all the time.

Even if the semantics isn't completely clear, we can just add a bunch of asserts to make sure only the things we can prove are done, everything else fails validation. In time, we can add more functionality upstream once we're clear on what is needed and how to implement it.

Waiting until we have a complete picture won't work, because we need to implement things to have a better picture and right now we can't implement all the things that everyone may one day want, because we don't know what it is, nor have the need for those things.

[rengolin]

https://reviews.llvm.org/rG9aa505a28d827f13ac9c6268f5834592b1a150e3 W00t!

Now we need to wait for the fallout, stabilise, and merge the changes, removing our local implementation.

[rengolin]

We've rebased on top of LLVM with that commit already, so just need to replace internally