[Bug target/70734] __builtin_add_overflow emits poor code on x86-64

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70734

--- Comment #7 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> It's not unused, it's stored to memory.

Of course, I read the testcase too quickly and thought *r was a throw-away
local variable... Sorry about that.