[Bug libstdc++/84949] -ffast-math bugged with respect to NaNs

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84949

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
std::numeric_limits<float> defines:

      static _GLIBCXX_USE_CONSTEXPR bool has_infinity = __FLT_HAS_INFINITY__;
      static _GLIBCXX_USE_CONSTEXPR bool has_quiet_NaN = __FLT_HAS_QUIET_NAN__;
      static _GLIBCXX_USE_CONSTEXPR bool has_signaling_NaN = has_quiet_NaN;
      static _GLIBCXX_USE_CONSTEXPR float_denorm_style has_denorm
        = bool(__FLT_HAS_DENORM__) ? denorm_present : denorm_absent;
      //...
      static _GLIBCXX_USE_CONSTEXPR bool is_iec559
        = has_infinity && has_quiet_NaN && has_denorm == denorm_present;

And that seems to be the right thing to do. If the compiler tells us the type
has infinities and NaNs then we expose that through std::numeric_limits.

I don't think we want the C++ library to be inconsistent with the compiler
here. So maybe any change should be in the compiler not libstdc++.