Add some alignment info where LLVM might otherwise be more pessimistic. In particular, %.nest_arg is always aligned even though it's bitcast from i8*. Pointers in vtables are also guaranteed to be stored at aligned addresses.

// testcase from bug #199

struct Color {
    uint c;

}

struct Vertex {
    Color c;
}

void main() {
    Color c = {0xffffffff};

    auto v = Vertex(c);

    assert(v.c.c == 0xffffffff);  // fails in LDC
}