Return the old value from atomic_fetch_op_explicit. (#640)

This matches the behavior of Metal's atomic_fetch_add_explicit and friends.

Author: maleadt

src/device/intrinsics/atomics.jl

@@ -112,7 +112,7 @@ end
         cmp = old
         new = convert(T, op(old, val))
         old = atomic_compare_exchange_weak_explicit(ptr, cmp, new)
-        isequal(old, cmp) && return new
+        isequal(old, cmp) && return old
     end
 end