RFR: 8316893: Compile without -fno-delete-null-pointer-checks [v3]

[Daniel Jeliński]

> Please review this patch that reenables `delete-null-pointer-checks` optimization in GCC and Clang.
> 
> It also disables `nonnull-compare` warning that is now triggered in debug builds. The problematic code will be addressed by #15927.
> 
> Mach5 Tier1-5 testing clean.

Daniel Jeliński has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Restore quotes
 - Remove no longer needed warning suppressions
 - Merge remote-tracking branch 'origin' into delete-nullpointer
 - Fix typo
 - Disable nonull-compare warnings
 - Remove -fno-delete-null-pointer-checks

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15982/files
  - new: https://git.openjdk.org/jdk/pull/15982/files/d4d79a81..6e5738b6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15982&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15982&range=01-02

  Stats: 1221 lines in 60 files changed: 675 ins; 224 del; 322 mod
  Patch: https://git.openjdk.org/jdk/pull/15982.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15982/head:pull/15982

PR: https://git.openjdk.org/jdk/pull/15982