Removed some checks for abstract llvm types that were too strict, a 'opaque* null' is a valid initializer... when structs in D can be just a forward reference.

module bug49;

struct S
{
    int i;
    long l;
}

void main()
{
    S s;
    s.i = 0x__FFFF_FF00;
    s.l = 0xFF00FF_FF00;
    s.i &= s.l;
}