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

The implementation only allows to bit-cast between two 0-D vectors. We could
probably support casting from/to vectors like `vector<1xf32>`, but I wasn't
convinced that this would be important and it would require breaking the
invariant that `BitCastOp` works only on vectors with equal rank.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D114854