RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v11]

[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:

  Add include to resolve undeclared enum

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/401/files
  - new: https://git.openjdk.org/shenandoah/pull/401/files/2a4c62c2..2cb710d0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=10
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=09-10

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