Clean up VectorType construction.

This CL introduces the following related changes:
- factor out element type validity checking to a static member function
  VectorType::isValidElementType;
- introduce get/getChecked similarly to MemRefType, where the checked function
  emits errors and returns nullptrs;
- remove duplicate element type validity checking from the parser and rely on
  the type constructor to emit errors instead.

PiperOrigin-RevId: 220693828