RFR: 8310574 GenShen: Should not update-references for in-place-promotions

[William Kemper]

When a cycle _only_ promotes regions in place, it is not necessary to update refs.

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

Commit messages:
 - Skip update-refs when collection set is empty but regions were promoted in place

Changes: https://git.openjdk.org/shenandoah/pull/291/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=291&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310574
  Stats: 32 lines in 4 files changed: 20 ins; 10 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/291.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/291/head:pull/291

PR: https://git.openjdk.org/shenandoah/pull/291