[Bug target/120083] Use vcvtneps2bf16 AVX512BF instruction to convert float to __bf16 on -O3 without needing -Ofast

[mcccs at gmx dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120083

--- Comment #4 from mcccs at gmx dot com ---
By the way Andrew, wouldn't it be more performant to check for sNaN then use
vcvtneps2bf16 instead of completely avoiding vcvtneps2bf16?