RFR: 8318019: GenShen: Fix assertion to allow empty evacuation cycles

[Kelvin Nilsen]

Assertion needs to allow for gc phase EVACUATION even if no forwarding objects because we are only promoting in place.

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Commit messages:
 - Fix typo and style issues
 - Relax forwarding assertion to account for promote-in-place evacuations

Changes: https://git.openjdk.org/shenandoah/pull/337/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=337&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318019
  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/337.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/337/head:pull/337

PR: https://git.openjdk.org/shenandoah/pull/337