RFR: JDK-8077842 - Remove the level parameter passed around in GenCollectedHeap

[Jesper Wilhelmsson]

Hi,

Please review the next cleanup in the generation cleanup series. This time the 
'level' is at stake. We pass it around in lots of places but in reality we 
already know which level (i.e. generation) we want to work with, so the level is 
just noise. In some places we want to verify that we are in the correct 
generation, or we pass a generation on to some generic code that can work on 
both young and old, in these cases we are now more explicit and makes it clear 
which generation we are dealing with instead of just sending a number.

Bug: https://bugs.openjdk.java.net/browse/JDK-8077842
Webrev: http://cr.openjdk.java.net/~jwilhelm/8077842/webrev.00/

Thanks,
/Jesper