[mlir][Vector] Allow a 0-d for for vector transfer ops.

[mlir][Vector] Allow a 0-d for for vector transfer ops.

This revision updates the op semantics, printer, parser and verifier to allow 0-d transfers.
Until 0-d vectors are available, such transfers have a special form that transits through vector<1xt>.
This is a stepping stone towards the longer term work of adding 0-d vectors and will help significantly reduce corner cases in vectorization.

Transformations and lowerings do not yet support this form, extensions will follow.

Differential Revision: https://reviews.llvm.org/D111559