RFR: JDK-8221766: Load-reference barriers for Shenandoah

[Roland Westrelin]

Hi Vladimir,

> opto/loopnode.cpp new is_Phi check was added. Please, explain.

When we expand barriers, if we find a null check nearby we move the
barrier close to the null check so there's a better chance of converting
it to an implicit null check. That happens as part of a pass of loop
opts. I think that's where that change comes from but I don't remember
the details. In general we need the control that's assigned to a load to
not be too conservative.

Anyway, that change is not required for correctness. But it looks
reasonable to me.

Roland.