missed optimization: combine writes to atomics

We should be able to combine atomic operations to the same address the same way we do with non-atomic operations.

https://godbolt.org/z/K54M5Y7e3

```
void foo(std::atomic& r) {
r.fetch_add(2, std::memory_order_relaxed);
r.fetch_add(4, std::memory_order_relaxed);
}

void foo2(std::atomic& r) {
r.fetch_add(6, std::memory_order_relaxed);
}

void bar(int& r) {
r += 2;
r += 4;
}

void bar2(int& r) {
r += 6;
}
```