RFR: Remove redundant null checks

[Vicente Romero]

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

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Commit messages:
 - regression tests
 - remove redundant null checks
 - Merge branch 'bworld' into remove.redundant.null.checks
 - remove redundant null checks

Changes: https://git.openjdk.org/valhalla/pull/1947/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1947&range=00
  Stats: 126 lines in 2 files changed: 111 ins; 0 del; 15 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