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

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM

This seems like a reasonable starting point. Something I'm a bit concerned about is that this may break DAGCombiner patterns that look for ZERO_EXTEND. Though given that RISCVCodeGenPrepare already does this replacement aggressively, I guess it's not a big issue in practice.

https://github.com/llvm/llvm-project/pull/70725