RFR: 8316893: Compile without -fno-delete-null-pointer-checks [v2]

[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 incrementally with one additional commit since the last revision:

  Fix typo

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15982/files
  - new: https://git.openjdk.org/jdk/pull/15982/files/b5aade6c..d4d79a81

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15982&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15982&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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