RFR: 8359126: [AIX] new test TestImplicitNullChecks.java fails

[Martin Doerr]

Switch off the IR rule which expects no explicit null check for a load operation for AIX. I had to enhance the IR code.

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

Commit messages:
 - 8359126: [AIX] new test TestImplicitNullChecks.java fails

Changes: https://git.openjdk.org/jdk/pull/25728/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25728&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359126
  Stats: 8 lines in 2 files changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25728.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25728/head:pull/25728

PR: https://git.openjdk.org/jdk/pull/25728