RFR: 8338535: GenShen: some style improvements to source code implementation [v4]

[Kelvin Nilsen]

> 1. Hide union data structure behind accessor methods for ShenandoahHeuristics::RegionData._u
> 2. Change type of ShenandoahCollectionSet::establish_preselected from byte* to byte[] to clarify intended usage.
> 3. Remove unhelpful comment from shenandoahFullGC.cpp

Kelvin Nilsen has updated the pull request incrementally with three additional commits since the last revision:

 - Remove deprecated code
 - Revert changes to declaration of RegionData array argument
   
   Use RegionData* rather than RegionData[] declaration style for
   consistency with other hotspot code.
 - Turn RegionData into a class and fix accessors

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/475/files
  - new: https://git.openjdk.org/shenandoah/pull/475/files/83e84fce..15cdbada

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=475&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=475&range=02-03

  Stats: 92 lines in 20 files changed: 10 ins; 7 del; 75 mod
  Patch: https://git.openjdk.org/shenandoah/pull/475.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/475/head:pull/475

PR: https://git.openjdk.org/shenandoah/pull/475