RFR: Remove redundant null checks [v6]

[Vicente Romero]

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

Vicente Romero has updated the pull request incrementally with two additional commits since the last revision:

 - addressing review comments
 - adding test cases

-------------

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1947/files
  - new: https://git.openjdk.org/valhalla/pull/1947/files/a4800d08..c8b43534

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1947&range=05
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1947&range=04-05

  Stats: 52 lines in 3 files changed: 37 ins; 3 del; 12 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