[lworld] RFR: 8339481: [lworld] C2: Rename InlineTypeNode::IsInit to NotNull

[Marc Chevalier]

Picked the proposed name I found the most fitting. Added a comment to address the concern in the issue.

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Commit messages:
 - Rename InlineTypeNode::IsInit to NullMarker, hopefully with helpful comment

Changes: https://git.openjdk.org/valhalla/pull/1491/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1491&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339481
  Stats: 14 lines in 2 files changed: 3 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/valhalla/pull/1491.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1491/head:pull/1491

PR: https://git.openjdk.org/valhalla/pull/1491