RFR: 8209831: ZGC: Clean up ZRelocationSetSelectorGroup::semi_sort()

[Per Liden]

A cleanup of ZRelocationSetSelectorGroup::semi_sort() can save us 16K 
worth of stack space, by having one single array act as both "slots" and 
"fingers". This works because the "slots" array is just an intermediate 
step when calculating the "fingers", and the different uses don't overlap.

Bug: https://bugs.openjdk.java.net/browse/JDK-8209831
Webrev: http://cr.openjdk.java.net/~pliden/8209831/webrev.0

/Per