[llvm] X86: Do not return invalid cost for fp16 conversion (PR #114128)

[via llvm-commits]

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

Thanks for the quick turnaround on this Matthias! This looks like it fixes the failure in jax/.../lax_test :)

https://github.com/llvm/llvm-project/pull/114128