RFR: 8223308: ZGC: Remove ZGranuleMap::size()

[Per Liden]

ZAddressOffsetMax is no longer a constant, so ZGranuleMap::size() will 
not be as nicely folded as it used to be. I suggest we instead calculate 
the size once and store it in a variable. This is not critical for 
performance, but I still think it's nice to do.

Bug: https://bugs.openjdk.java.net/browse/JDK-8223308
Webrev: http://cr.openjdk.java.net/~pliden/8223308/webrev.0

/Per