[mlir][test] Add TD example for peel+vectorize (depthwise conv) (#90200)

Adds an example that combines loop peeling and scalable vectorisation of
`linalg.depthwise_conv_2d_nhwc_hwc`. This is similar to
transform-op-peel-and-vectorize.mlir and is meant to demonstrate how to
avoid masking when vectorising using scalable vectors.