RFR: Only register retired plab waste in old regions

[William Kemper]

This is a follow up to [PR 74](https://github.com/openjdk/shenandoah/pull/74) in which we register filler objects from retired gclabs. This should _only_ be done if the plab was in an old region. Registering young objects in the remembered set confuses the verifier.

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

Commit messages:
 - Only register retired plab waste in old regions

Changes: https://git.openjdk.java.net/shenandoah/pull/77/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=77&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/77.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/77/head:pull/77

PR: https://git.openjdk.java.net/shenandoah/pull/77