RFR: 8221540: ZGC: Reduce width of zForwardingEntry::from_index field

[Per Liden]

The zForwardingEntry::from_index field only needs 18 bits, as it only 
needs to be able to represent the largest object index, which is:

ZPageSizeSmall / MinObjectAlignment == 2M / 8 == 262144 == 1 << 18.

Shrinking the from_index allows us to grow the to_offset, which in turn 
allows us to increase the max heap size in the future. However, with 
these changes, we can no longer assume that (uint64_t)-1 is an empty 
entry. So we need to introduce a new flag to indicate if an entry is 
populated or not.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221540
Webrev: http://cr.openjdk.java.net/~pliden/8221540/webrev.0

/Per