[mlir][Linalg] Refactor conv vectorization to decouple memory from vector ops.

This refactoring prepares conv1d vectorization for a future integration into
the generic codegen path.
Once transfer_read / transfer_write vectorization also supports sliding windows,
the special pattern for conv can disappear.
This will also likely need a vector.conv operation.

Differential Revision: https://reviews.llvm.org/D112797