Operation-assignment operators don't work with vector/SIMD types

Repro:

```c3
module test_branch;
import std::io;

typedef Mm512i = long[<8>] @simd;

fn void main()
{
    Mm512i v, x;
    v[..] = 7;
    x[..] = 5;
    // v += x;   // does not work, but should
    v = v + x;   // works
    io::printfn("RESULT: %s", v); //should print 12s
}
```