[MLIR] Allow empty vector type attributes

[rriddle]

This changes the definition of the type. The commit should have a bit more context, and also update documentation (assuming this is something we actually want to do).

[frgossen]

Thanks!
After reading the documentation, I think I was just using the wrong thing.

mlir/lib/IR/BuiltinTypes.cpp

 LogicalResult VectorType::verify(function_ref<InFlightDiagnostic()> emitError,
                                  ArrayRef<int64_t> shape, Type elementType,
                                  unsigned numScalableDims) {
   if (!isValidElementType(elementType))
     return emitError()
            << "vector elements must be int/index/float type but got "
            << elementType;
 
-  if (any_of(shape, [](int64_t i) { return i <= 0; }))
+  if (any_of(shape, [](int64_t i) { return i < 0; }))
     return emitError()
-           << "vector types must have positive constant sizes but got "
+           << "vector types must have non-negative constant sizes but got "
            << shape;
 
   return success();
 }
 
 VectorType VectorType::scaleElementBitwidth(unsigned scale) {
   if (!scale)
     return VectorType();