[mlir][linalg] Refactor convolution to img2col conversion to use gather semantics

Following up on the comments in https://reviews.llvm.org/D144108 this
patch refactors the im2col conversion patterns for `linalg.conv_2d_nhwc_hwcf`
and `linalg.conv_2d_nchw_fchw` convolutions to use gather semantics for the im2col
packing `linalg.generic`.

Follow up work can include a similar pattern for depthwise convolutions
and a generalization of the patterns here to work with any `LinalgOp` as
well.

Differential Revision: https://reviews.llvm.org/D144678