RFR: Remove redundant null checks [v5]

[Vicente Romero]

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

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

  adding more tests

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1947/files
  - new: https://git.openjdk.org/valhalla/pull/1947/files/64e65e71..a4800d08

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1947&range=04
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1947&range=03-04

  Stats: 69 lines in 1 file changed: 56 ins; 1 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