RFR: JDK-8300243: Replace NULL with nullptr in share/compiler/

[Johan Sjölen]

Do the conversion in the share/compiler/ sub-directory and all of its files.

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Commit messages:
 - Manual fixes
 -  Replace NULL with nullptr in share/compiler/

Changes: https://git.openjdk.org/jdk/pull/12032/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12032&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300243
  Stats: 594 lines in 27 files changed: 0 ins; 0 del; 594 mod
  Patch: https://git.openjdk.org/jdk/pull/12032.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12032/head:pull/12032

PR: https://git.openjdk.org/jdk/pull/12032