[mlir] Add support for parallel dim *after* reduction dim in split reduction

Previously, splitReduction transformation added the split parallel dimension
*before* the reduction dimension, leading to tiling for reduction. This
commit creates an option to create the parallel dimension *after* the
reduction dimension, allowing us to transform the op into vertical reduction
with SIMD parallelism.

Reviewed By: ThomasRaoux, dcaballe

Differential Revision: https://reviews.llvm.org/D134764