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.

interface I {}
class A : I {}
class B : A {}

void main () {
    A a = new A;
    I i = a;

    assert(!cast(B)a);
    assert(!cast(B)i);
}