[mlir] fix out-of-bounds in reduction tiling

A transformation tiling a reduction dimension of a Linalg op needs a
tile size for said dimension. When an insufficient number of dimensions
was provided, it would segfault due to out-of-bounds access to a vector.

Also fix incorrect error reporting in the structured transform op
exercising this functionality.

Reviewed By: springerm, ThomasRaoux

Differential Revision: https://reviews.llvm.org/D141046