RFR: 8256392: C2: Various Node cleanups

[Claes Redestad]

- Unused methods:
 Node::walk, walk_, nop, lookup, rpop
 Node_Array::reset, sort
- Disallowing 0 as values to OptoNodeListSize and OptoBlockListSize can simplify some logic: e.g., Node_Array::_max > 0 becomes an invariant (with reset unused and removed)

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Commit messages:
 - Merge branch 'master' into node_cleanup
 - Merge branch 'master' into node_cleanup
 - Remove unused code
 - More cleanups
 - Merge branch 'master' into node_cleanup
 - Remove unused code in node

Changes: https://git.openjdk.java.net/jdk/pull/1223/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1223&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256392
  Stats: 116 lines in 3 files changed: 11 ins; 65 del; 40 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1223.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1223/head:pull/1223

PR: https://git.openjdk.java.net/jdk/pull/1223