RFR: 8338535: GenShen: some style improvements to source code implementation [v3]

[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 one additional commit since the last revision:

  Code touchup
  
  Remove 2 unneeded blank lines
  Move and simplify a comment from call point to called method
  implementation

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/475/files
  - new: https://git.openjdk.org/shenandoah/pull/475/files/7fa4a49c..83e84fce

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=475&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=475&range=01-02

  Stats: 6 lines in 4 files changed: 1 ins; 5 del; 0 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