[mlir][linalg] Rewrite `linalg.conv_2d_nhwc_hwcf` into 1-D

We'd like to take a progressive approach towards Fconvolution op
CodeGen, by 1) tiling it to fit compute hierarchy first, and then
2) tiling along window dimensions with size 1 to reduce the problem
to be matmul-like. After that, we can 3) downscale high-D convolution
ops to low-D by removing the size-1 window dimensions. The final
step would be 4) vectorizing the low-D convolution op directly.

We have patterns for 1), 2), and 4). This commit adds a pattern for
3) for `linalg.conv_2d_nhwc_hwcf` ops as a starter. Supporting other
high-D convolution ops should be similar and mechanical.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D112928