[mlir][vector] `LoadOp`/`StoreOp`: Allow 0-D vectors (#76134)

Similar to `vector.transfer_read`/`vector.transfer_write`, allow 0-D
vectors.

This commit fixes
`mlir/test/Dialect/Vector/vector-transfer-to-vector-load-store.mlir`
when verifying the IR after each pattern (#74270). That test produces a
temporary 0-D load/store op.