RFR: 8335663: Fix simple -Wzero-as-null-pointer-constant warnings in C2 code

[Kim Barrett]

Please review this trivial change that replaces some uses of literal 0 as a null
pointer constant in C2 code to instead use nullptr.

Testing: mach5 tier1

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Commit messages:
 - fix labelOper ctor
 - fix PhiNode::pinned
 - fix Block ctor
 - fix ConstantTable::Constant ctor

Changes: https://git.openjdk.org/jdk/pull/20022/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20022&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335663
  Stats: 7 lines in 4 files changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/20022.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20022/head:pull/20022

PR: https://git.openjdk.org/jdk/pull/20022