[SelectionDAG] Add initial support for nneg flag on ISD::ZERO_EXTEND. (#70872)

This adds the nneg flag to SDNodeFlags and the node printing code.
SelectionDAGBuilder will add this flag to the node if the target doesn't
prefer sign extend.

A future RISC-V patch can remove the sign extend preference from
SelectionDAGBuilder.

I've also added the flag to the DAG combine that converts
ISD::SIGN_EXTEND to ISD::ZERO_EXTEND.