[Bug target/68908] inefficient code for _Atomic operations

[rth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68908

--- Comment #4 from Richard Henderson <rth at gcc dot gnu.org> ---
I think we should rather handle this in the front end than with
quite complex pattern matching.

If we want to do any complex logic with atomics in the middle-end,
we should change their representation so that we don't have to
struggle with a sequence of builtins.  Which is clearly a subject
for gcc7 at minimum.