RFR: 8303252: G1: Return early from Full-GC if no regions are selected for compaction.

[Ivan Walulya]

Please review this change to return early from full-gc in case no regions have been selected as targets for compaction. 
To skip  `phase3_adjust_pointers` and `phase4_do_compaction()`, we need to decouple cleanup work that was piggybacked onto these phases. Thus, the `phase5_` is introduced to do the cleanup.

Testing: Tier 1-3.

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Changes: https://git.openjdk.org/jdk/pull/12768/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12768&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303252
  Stats: 342 lines in 9 files changed: 214 ins; 122 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/12768.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12768/head:pull/12768

PR: https://git.openjdk.org/jdk/pull/12768