Issue 13044 - Assignment of structs with const members

https://issues.dlang.org/show_bug.cgi?id=13044
No. If member-wise assignment is used, and members have const opAssign, it would be called.

from test6216d() in runnable/assignable.d

```d
    static struct X
    {
        int[] arr;
        void opAssign(const X rhs) const { ++cnt; }
    }
    static struct S
    {
        int n;
        const(X) x;
    }
    S s;
    s = s;   // assignable
```

Therefore, `isAssignable()` should be checked only in copy-and-swap assignment branch.