[Bug other/63387] Optimize pairs of isnan() calls into a single isunordered()

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63387

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---
According to Joseph's explanation at
https://sourceware.org/bugzilla/show_bug.cgi?id=17441 , this transformation
should be conditional to !flag_signaling_nans.