[mlir][Vector] Improve `vector.mask` verifier (#139823)
This PR improves the `vector.mask` verifier to make sure it's not
applying masking semantics to operations defined outside of the
`vector.mask` region. Documentation is updated to emphasize that and
make it clearer, even though it already stated that.
As part of this change, the logic that ensures that a terminator is
present in the region mask has been simplified to make it less
surprising to the user when a `vector.yield` is explicitly provided in
the IR.
