RFR: 8284584: Avoid duplicate node_idx_t definitions

[Aleksey Shipilev]

While looking at `Node`, I realized there are two definitions for node_idx_t, another one added by [JDK-8076284](https://bugs.openjdk.java.net/browse/JDK-8076284). These should be de-duplicated to avoid confusion.

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Commit messages:
 - Move

Changes: https://git.openjdk.java.net/jdk/pull/8164/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8164&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284584
  Stats: 12 lines in 3 files changed: 5 ins; 7 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8164.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8164/head:pull/8164

PR: https://git.openjdk.java.net/jdk/pull/8164