RFR: 8254269: simplify Node::disconnect_inputs

[Xin Liu]

8254269: simplify Node::disconnect_inputs

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Commit messages:
 - 8254269: simplify Node::disconnect_inputs

Changes: https://git.openjdk.java.net/jdk/pull/589/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=589&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254269
  Stats: 49 lines in 11 files changed: 1 ins; 9 del; 39 mod
  Patch: https://git.openjdk.java.net/jdk/pull/589.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/589/head:pull/589

PR: https://git.openjdk.java.net/jdk/pull/589