RFR: 8303252: G1: Return early from Full-GC if no regions are selected for compaction. [v2]

[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.

Ivan Walulya has updated the pull request incrementally with one additional commit since the last revision:

  Thomas review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12768/files
  - new: https://git.openjdk.org/jdk/pull/12768/files/06da84e1..21486e0c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12768&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12768&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 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