RFR: Remove redundant null checks [v9]

[Vicente Romero]

> some of the null checks javac generates are redundant and can be removed

Vicente Romero has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 14 commits:

 - fixes needed after merge
 - merge with bworld
 - Merge branch 'bworld' into remove.redundant.null.checks
 - minor change
 - addressing review comments
 - adding test cases
 - adding more tests
 - adding more tests
 - minor change
 - another change
 - ... and 4 more: https://git.openjdk.org/valhalla/compare/636ee540...c0d70e69

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Changes: https://git.openjdk.org/valhalla/pull/1947/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1947&range=08
  Stats: 332 lines in 4 files changed: 274 ins; 11 del; 47 mod
  Patch: https://git.openjdk.org/valhalla/pull/1947.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1947/head:pull/1947

PR: https://git.openjdk.org/valhalla/pull/1947