Fuse multiplication by a splat into conv2d and similar ops

_Cf. the comment thread by @mdgrs in https://github.com/google/heir/pull/3006#discussion_r3325235127_

https://github.com/google/heir/pull/3006 implemented fusion of batchnorm1d into preceding convolution and linear ops. The operations that batchnorm1d lowers to are multiplication by general tensors, which only make sense to fuse into channel-wise operations.

However, some constructions (reportedly average pooling and sum pooling) reduce to multiplication by a splat after a non-channel ops like conv2d and conv1d. These splats can be fused into the filter matrix of the convolution. This issue is to add support for that.