[Bug target/120083] New: Use vcvtneps2bf16 AVX512BF instruction to convert float to __bf16

[mcccs at gmx dot com via Gcc-bugs]

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

            Bug ID: 120083
           Summary: Use vcvtneps2bf16 AVX512BF instruction to convert
                    float to __bf16
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mcccs at gmx dot com
  Target Milestone: ---

pr119626 was for arm, this is for x86.

Copy LLVM 21's behavior:

With -mavx512vl -mavx512bf16 and -O3

__bf16 cvt(float r) {
    return (__bf16) r;
}

should use vcvtneps2bf16

related: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117542