[mlir][Linalg] Break unnecessary dependency through unused `outs` tensor.

LinalgOps that are all parallel do not use the value of `outs`
tensor. The semantics is that the `outs` tensor is fully
overwritten. Using anything other than `init_tensor` can add false
dependencies between operations, when the use is just for the shape of
the tensor. Adding a canonicalization to always use `init_tensor` in
such cases, breaks this dependence.

Differential Revision: https://reviews.llvm.org/D102561