RFR: 8276921: G1: Remove redundant failed evacuation regions calculation in RemoveSelfForwardPtrHRClosure

[Hamlin Li]

This is a minor optimization.
We already have the number of failed evacuation region numbers, there is no need to calculate it on fly, at least in product code, so make it debug only, and add assert to verify the result.

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Commit messages:
 - Remove dead code
 - use COMMA
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6362/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6362&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276921
  Stats: 5 lines in 1 file changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6362.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6362/head:pull/6362

PR: https://git.openjdk.java.net/jdk/pull/6362