RFR: 8338535: GenShen: some style improvements to source code implementationle [v2]

[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 name of zero_RegionData function

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/475/files
  - new: https://git.openjdk.org/shenandoah/pull/475/files/45057ce5..7fa4a49c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=475&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=475&range=00-01

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 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