RFR: 8252505: C1/C2 compiler support for blackholes [v11]

[Vladimir Ivanov]

> So where do we go from here? Somehow recording the operands of 
> MachCallBlackhole?

Have you tried `PhaseRegAlloc::dump_register(const Node *n, char *buf)` 
on argument inputs?

Best regards,
Vladimir Ivanov