Allow element type traits to operate on scalars

This allows confirming that a scalar argument has the same element type as a shaped one. It's easy to validate a type is shaped on its own if that's desirable, so this shouldn't make that use case harder. This matches the behavior of other traits that operate on element type (e.g. AllElementTypesMatch). Also this makes the code simpler because now we just use getElementTypeOrSelf.

Verified that all uses in core already check the type is shaped in another way.

PiperOrigin-RevId: 273068507