RFR: 8338535: GenShen: some style improvements to source code implementation [v5]

[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:

  Fix whitespace

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/475/files
  - new: https://git.openjdk.org/shenandoah/pull/475/files/15cdbada..b6f06a4a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=475&range=04
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=475&range=03-04

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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