[mlir][linalg] Promote operands for convolution vectorization

We are already doing this for depthwise convolution and pooling.
This helps to preserve the promotion semantics from Linalg op
definitions to lower layers.

Along the way, fixed the type mismatch issue in the existing
`promote` implementation.

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D148471