RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v7]

[Kelvin Nilsen]

> When adjusting LAB sizes, round down rather than rounding up.  Otherwise, we may violate the ShenandoahHumongousThreshold bound.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Fix up a few other places where PLAB min size or current size gets set
  
  This may explain how unaligned sizes were being detected by existing
  assertions.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/401/files
  - new: https://git.openjdk.org/shenandoah/pull/401/files/66609d9b..77663512

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=06
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=05-06

  Stats: 9 lines in 3 files changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/401.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/401/head:pull/401

PR: https://git.openjdk.org/shenandoah/pull/401