[mlir][linalg] Padding transformation: Write back result to original destination

Copy back the padded result to the original destination of the computation. This is important for bufferization, to ensure that the result of the computation does not suddenly materialize in a different buffer due to padding.

A `bufferization.copy_tensor` is inserted for every (unpadded) result. Such ops bufferize to memcpys, but they fold away, should the padding fold away.

Differential Revision: https://reviews.llvm.org/D153554