RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v5]

[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:

  Do not align_up size of object allocated in plab slow
  
  Align the plab size, but not the size of the object to be allocated within the plab.
  If we increase the size of the allocated object, this creates confusion in that the
  object's size reported by its header does not span all of the bytes allocated for the
  object, so the memory is not parsable during remembered set scanning.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/401/files
  - new: https://git.openjdk.org/shenandoah/pull/401/files/5de18812..45695f04

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=04
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=03-04

  Stats: 39 lines in 2 files changed: 9 ins; 17 del; 13 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