[mlir][Vector] Support 0-D vectors in `ConstantMaskOp`

To support creating both a mask with just a single `true` and `false` values,
I had to relax the restriction in the verifier that the rank is always equal to
the length of the attribute array, in other words, we now allow:

- `vector.constant_mask [0] : vector<i1>` which gets lowered to
  `arith.constant dense<false> : vector<i1>`
- `vector.constant_mask [1] : vector<i1>` which gets lowered to
  `arith.constant dense<true> : vector<i1>`

(the attribute list for the 0-D case must be a singleton containing
either `0` or `1`)

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D115023