[SDAG] Prefer forming sign_extend for zext nneg per target preference (#70725)

Builds on #67982 which recently introduced the nneg flag on a zext
instruction. Note that this change is the first point where the flag is
being used for an optimization, and thus may expose latent miscompiles.
We've recently taught both CVP and InstCombine to infer the flag when
forming zext, but nothing else is using the flag just yet.