[TableGen] Model variadic operands using Variadic<Type>

Previously, we were using the trait mechanism to specify that an op has variadic operands.
That led a discrepancy between how we handle ops with deterministic number of operands.
Besides, we have no way to specify the constraints and match against the variadic operands.

This CL introduced Variadic<Type> as a way to solve the above issues.

PiperOrigin-RevId: 232656104