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 complex3;

void main()
{
    cfloat c1 = 1f + 0i;
    cfloat c2 = 0f + 0i;
    c2 += 1f + 0i;
    assert(c1 == c2);
}