[mlir][linalg] Changing the positions of introduced parallel loop in...

[mlir][linalg] Changing the positions of introduced parallel loop in SplitReduction to be consistent with IREE's downstream passes

IREE's passes depend on the behavior of SplitReduction's introduced
parallel loop being the same as the introduced dimension in the
intermediate tensor (the order of loops was changed in
https://reviews.llvm.org/D137478).

Differential Revision: https://reviews.llvm.org/D138972