RFR: 8338535: GenShen: some style improvements to source code implementationle

[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

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Commit messages:
 - Remove unhelpful comment from shenandoahFullGC.cpp
 - Use accessor methods and tagged access for more maintainable code
 - Change type declaration for establish_preselected argument
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - ... and 13 more: https://git.openjdk.org/shenandoah/compare/7cc63abc...45057ce5

Changes: https://git.openjdk.org/shenandoah/pull/475/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=475&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338535
  Stats: 118 lines in 22 files changed: 62 ins; 5 del; 51 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