RFR: Generate missing null checks [v6]

[Vicente Romero]

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

Vicente Romero has updated the pull request incrementally with one additional commit since the last revision:

  review comments

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1867/files
  - new: https://git.openjdk.org/valhalla/pull/1867/files/b6032148..e1f24e4e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1867&range=05
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1867&range=04-05

  Stats: 24 lines in 2 files changed: 4 ins; 12 del; 8 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