[MLIR] Drop strict super-vector requirement in MaterializeVector

The strict requirement (i.e. at least 2 HW vectors in a super-vector) was a
premature optimization to avoid interfering with other vector code potentially
introduced via other means.

This CL avoids this premature optimization and the spurious errors it causes
when super-vector size == HW vector size (which is a possible corner case).

This may be revisited in the future.

PiperOrigin-RevId: 227763966