[Bug c++/98348] GCC 10.2 AVX512 Mask regression from GCC 9

[crazylht at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98348

--- Comment #12 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Jakub Jelinek from comment #11)
> I'm not sure about the knot changes, isn't that too risky at least at this
> point?
> I mean, can't we instead just match what knot emits?
> 
As indicated in
https://gcc.gnu.org/pipermail/gcc-patches/2020-August/552205.html, we support
bitwise operator for avx512 masks, so i believe the change is fine for GCC11,
but not suitable for GCC10. 

For backport convenience, I'll make a version that matches the knot.

> As for the new predicate, I think we should check CONST_DOUBLE_AS_FLOAT_P
> (op)
> before trying to do the lowpart_subreg, perhaps even check if the
> CONST_DOUBLE is NaN if it can be done cheaply before doing the more
> expensive lowpart_subreg.

Yes.