RFR: 8344321: GenShen: Fix various sonar scan warnings

[William Kemper]

Various sonar warnings are fixed: 
* unused variables removed
* implicit casts made explicit
* members are all initialized

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

Commit messages:
 - Merge remote-tracking branch 'shenandoah/master' into fix-sonar-findings
 - Use checked_cast in more places, fix usage error in another
 - Bundle of sonar fixes

Changes: https://git.openjdk.org/shenandoah/pull/544/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=544&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344321
  Stats: 53 lines in 9 files changed: 1 ins; 31 del; 21 mod
  Patch: https://git.openjdk.org/shenandoah/pull/544.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/544/head:pull/544

PR: https://git.openjdk.org/shenandoah/pull/544