[mlir][Vector] Fold transpose splat to splat with transposed type.

This revision folds transpose splat to a new splat with the transposed vector type. For a splat, there is no need to actually do transpose for it, it would be more effective to just build a new splat as the result.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D123765