RFR: 8303252: G1: Return early from Full-GC if no regions are selected for compaction. [v3]

[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:

  Albert review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12768/files
  - new: https://git.openjdk.org/jdk/pull/12768/files/21486e0c..695f2bd7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12768&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12768&range=01-02

  Stats: 2 lines in 1 file changed: 2 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