[mlir][LinAlg][Transform] Add a transform op for conv2d to im2col

This patch adds patterns to convert `linalg.conv_2d_xxx` operations
into `linalg.generic` (for img2col packing) and `linalg.matmul`.

The meat of the patch comes straight from IREE
(https://github.com/iree-org/iree).
(To the original authors are you okay with that?)

What this patch adds is proper plumbing of the im2col patterns into the
transform dialect.

PS: Feel free to add more reviewers. I wanted to cover the original contributors of im2col in IREE but I'm not sure I got all of them.

Reviewed By: nicolasvasilache, ThomasRaoux

Differential Revision: https://reviews.llvm.org/D144108