RFR: Generate missing null checks

[Vicente Romero]

generate missing null checks like for method arguments, return arguments, etc

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Commit messages:
 - changes to test
 - Merge branch 'generate.missing.null.checks' of https://github.com/vicente-romero-oracle/valhalla into generate.missing.null.checks
 - recent changes
 - recent changes
 - Merge branch 'bworld' into generate.missing.null.checks
 - more changes
 - Merge branch 'generate.missing.null.checks' of https://github.com/vicente-romero-oracle/valhalla into generate.missing.null.checks
 - additional changes
 - additional changes
 - Merge branch 'bworld' into generate.missing.null.checks
 - ... and 1 more: https://git.openjdk.org/valhalla/compare/1d5dfc24...b676689c

Changes: https://git.openjdk.org/valhalla/pull/1867/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1867&range=00
  Stats: 376 lines in 6 files changed: 233 ins; 119 del; 24 mod
  Patch: https://git.openjdk.org/valhalla/pull/1867.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1867/head:pull/1867

PR: https://git.openjdk.org/valhalla/pull/1867