Use dag instead of list for operands to allow named operands.

Named operands allow generating builders with more meaningful names + lay the groundwork for allowing the specification of attributes as part of the inputs pattern of an op (which allows the declarative pattern rewrite generator to define ops with attributs). This is a minimal change that just changes how input operands are represented, changes to attributes in follow up and returnTypes later.

PiperOrigin-RevId: 225509805