[llvm] [X86] Fix miscompile of fptosi.sat.iN.f16 for NaN under avx512fp16 (PR #210556)

[Nathaniel McCallum via llvm-commits]

npmccallum wrote:

@RKSimon would you be able to add the **LLVM 23.x Release** milestone here? My `/cherry-pick` above failed with "missing milestone," and I don't have the permissions to set it.

I'd like to nominate this for 23.x: it's a silent miscompile (NaN → `iN::MIN` instead of `0` for signed saturating `f16`→int under avx512fp16), the fix is self-contained, and it cherry-picks cleanly onto `release/23.x` (the pre-image matches verbatim). Downstream Rust is affected on current nightly and is tracking it as well. Happy to re-run `/cherry-pick` once the milestone is set, or you can trigger it — whichever is easier.

https://github.com/llvm/llvm-project/pull/210556