[mlir][ArmSME] Use vector.reduction add in zero test

The inner 1d vector row can be summed with vector.reduction op. The
earlier mul reduction can't be updated similarly as it currently crashes
in the backend with:

  LLVM ERROR: Expanding reductions for scalable vectors is undefined.

Reviewed By: awarzynski, dcaballe

Differential Revision: https://reviews.llvm.org/D156701