[patch] Add an if-exists-then-else spec function

[Olivier Hainque]

Hello Armin,

> On 1 Oct 2020, at 18:20, Armin Brauns via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> 
> 
> could you please make sure to update the documentation around gcc/doc/invoke.texi:31574 accordingly?

Oh, sure.

> There's already a pending patch to make it more complete at https://gcc.gnu.org/pipermail/gcc-patches/2020-September/553321.html, but there shouldn't be any major conflicts between the two.

Understood, thanks for the heads-up.

I’ll post an updated version once I’m
done retesting it together with a couple of changes
addressing comments on other patches.

Thanks for your feedback!

Olivier