RFR: Generate missing null checks [v5]

[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:

  last iteration

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1867/files
  - new: https://git.openjdk.org/valhalla/pull/1867/files/ef7e0a7d..b6032148

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1867&range=04
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1867&range=03-04

  Stats: 31 lines in 5 files changed: 8 ins; 6 del; 17 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