[Bug target/79173] add-with-carry and subtract-with-borrow support (x86_64 and others)

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79173

--- Comment #4 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
Also, make sure that the optimization is still done when a variable is a
constant or replaced by a constant (with Clang, the optimization is no longer
done in such a case).