RFR: 8374149: [bworld] Define and use new null checking API [v4]

[Vicente Romero]

> Definition and use of a new null checking API

Vicente Romero 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 nine additional commits since the last revision:

 - Merge branch 'bworld' into JDK-8374149
 - adding a flag
 - additional changes
 - minor changes
 - additional changes to Checks
 - Merge branch 'bworld' into JDK-8374149
 - additional changes
 - Merge branch 'bworld' into JDK-8374149
 - 8374149: [bworld] Define and use new null checking API

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1859/files
  - new: https://git.openjdk.org/valhalla/pull/1859/files/43b44fff..c305e470

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1859&range=03
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1859&range=02-03

  Stats: 154 lines in 9 files changed: 104 ins; 38 del; 12 mod
  Patch: https://git.openjdk.org/valhalla/pull/1859.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1859/head:pull/1859

PR: https://git.openjdk.org/valhalla/pull/1859