RFR: Coalesce and fill old, pinned regions after old gen final mark

[William Kemper]

Pinned regions were not being coalesced and filled. This fixes a crash observed with the specjbb 2015 benchmark.

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Commit messages:
 - remove TODO comment
 - Coalesce and fill old, pinned regions after old gen final mark

Changes: https://git.openjdk.org/shenandoah/pull/149/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=149&range=00
  Stats: 13 lines in 1 file changed: 13 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/149.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/149/head:pull/149

PR: https://git.openjdk.org/shenandoah/pull/149