lower tensor::gather to linalg

[ziereis]

Hello,

after stumbling into an issue where iree couldn't lower some tensor ops, i asked about it on Discord and there has been a remark made that adding a lowering for these ops to linalg would be a good idea.

So I thought I could try doing so. I have no idea if what I'm doing is totally wrong so I'm happy about any feedback.

For now, I have just added one pattern to convert the tensor::gatherOp to a linalg generic. But it still needs a lot more tests so it's still a work in progress.