RFR: 8316893: Compile without -fno-delete-null-pointer-checks

[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.

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Commit messages:
 - Disable nonull-compare warnings
 - Remove -fno-delete-null-pointer-checks

Changes: https://git.openjdk.org/jdk/pull/15982/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15982&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316893
  Stats: 19 lines in 2 files changed: 2 ins; 14 del; 3 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