multimedia/libvpx cause clang fatal error on CURRENT

[Iblis Lin]

Hi,

I got this while building multimedia/libvpx

```
cc  -O2 -pipe -march=native  ...
#0: t25: v8i64 = llvm.x86.avx512.psad.bw.512 TargetConstant:i64<5820>, t11, t5

fatal error: error in backend: Do not know how to split the result of this operator!

cc: error: clang frontend command failed with exit code 70 (use -v to see invocation)
FreeBSD clang version 6.0.0 (branches/release_60 321788) (based on LLVM 6.0.0)
Target: x86_64-unknown-freebsd12.0
Thread model: posix
InstalledDir: /usr/bin
```

My box:

FreeBSD abeing 12.0-CURRENT FreeBSD 12.0-CURRENT #0 r328198: Mon Jan 22 13:26:48
CST 2018 root at abeing:/usr/obj/usr/src/amd64.amd64/sys/GENERIC  amd64

Here is the script
https://gist.github.com/iblis17/6a1d899462ba8ba1a475eebad83e993d

-- 
Iblis Lin
林峻頤