[Bug target/70421] [5/6 Regression] wrong code with v16si vector and useless cast at -O -mavx512f

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70421

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Zdenek Sojka from comment #3)
> Is the fact that the useless cast in:
>   x ^= ((v16si)v)[u[0]];
> changes the generated code (makes it significantly worse at all opt levels)
> worth creating a PR?

I think so.  But it is likely not a regression, so probably has to wait till
GCC 7.