RFR: 8342730: Get rid of SummaryDiff in VMATree [v3]

[Afshin Zafari]

> `SummaryDiff` result of any `register_mapping()` call, is passed into the function rather than returned. This enables avoiding copying the structure to/from the stack.
> 
> Tests:
> local linux-x64-debug: NMT gtests and JTREGs passed.

Afshin Zafari has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge remote-tracking branch 'origin/master' into _8342730_summary_diff_ref
 - better naming for diff
 - 8342730: Get rid of SummaryDiff in VMATree

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26761/files
  - new: https://git.openjdk.org/jdk/pull/26761/files/9b53507d..5178e1bb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26761&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26761&range=01-02

  Stats: 230268 lines in 3499 files changed: 173245 ins; 37169 del; 19854 mod
  Patch: https://git.openjdk.org/jdk/pull/26761.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26761/head:pull/26761

PR: https://git.openjdk.org/jdk/pull/26761