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

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

You need to update the SDAG canCreateUndefOrPoison() implementation to account for the new flag.

This generally looks ok to me, but I'll leave it to people who do more backend work to decide whether having this flag in SDAG is worthwhile. Given how target-specific SDAG is, I could see the argument for just using SEXT based on target preference and not adding the complexity of additional poison flags.

https://github.com/llvm/llvm-project/pull/70872