[CodeGen] Disable aggressive structor optimizations at -O0, take 3
The previous version of this patch (r332839) was reverted because it was
causing "definition with same mangled name as another definition" errors
in some module builds. This was caused by an unrelated bug in module
importing which it exposed. The importing problem was fixed in r336240,
so this recommits the original patch (r332839).
Differential Revision: https://reviews.llvm.org/D46685