[mlir][linalg] Downscale 2D convolution with unit dimensions to 1D convolution

Decompose conv_2d -> conv_1d.

This MR follows a similar approach to https://reviews.llvm.org/D112928.

This patch adds support to convert conv_2D operation with either unit height or unit width to conv_1D operation.

This is useful when 2D convolution is tiled to have a single dimension for either height or width and then can be vectorized once it is decomposed into 1D convolution.

This patch https://reviews.llvm.org/D145160 adds vector support for linalg.conv_1d operation and thereby allowing us to vectorize linalg.conv_2d operation after proper tiling.

This missing feature is reported here: https://discourse.llvm.org/t/vectorization-of-convolution-op/60458.

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D145162