RFR: enqueue barrier + some other things

[Roland Westrelin]

> one thing:
>
> doesn't static-null-checking like this:
>
>   if (val_t->meet(TypePtr::NULL_PTR) == val_t) {
>
> require remove_speculative() or such?

That's true only until remove_speculative_types() is called in
Compile::Optimize(), way before barriers are expanded.

Roland.