RFR(S) 8023691: Create interface for nodes in class Block

[Niclas Adlertz]

Hi all,

This is the first of many cleanups in class Block.
A review would be much appreciated. 

WEBREV: http://cr.openjdk.java.net/~adlertz/JDK-8023691/webrev00/
JBS: https://bugs.openjdk.java.net/browse/JDK-8023691

Kind Regards,
Niclas Adlertz