RFR (M): 8136681: Factor out IHOP calculation from G1CollectorPolicy

[Thomas Schatzl]

Hi,

  some comment about the logging: since unified logging has not been
finished yet, it adds logs using -XX:+PrintAdaptiveSizePolicy, even
adding a new category of messages.

There is a CR to adapt these messages to unified logging already
(JDK-8140777), pending on the timing with gc support for unified
logging.

Thanks,
  Thomas