RFR: Fix error in alignment of plabs

[Kelvin Nilsen]

A recent test run reported an assertion failure because of the error corrected in this patch.

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Commit messages:
 - Merge remote-tracking branch 'GitFarmBranch/fix-error-in-alignment-of-plabs' into fix-error-in-alignment-of-plabs
 - Fix alignment miscalculation for plabs

Changes: https://git.openjdk.org/shenandoah/pull/229/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=229&range=00
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/229.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/229/head:pull/229

PR: https://git.openjdk.org/shenandoah/pull/229