RFR: 8284584: Avoid duplicate node_idx_t definitions [v2]

[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.

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Leave one definition in compile.hpp only
 - Merge branch 'master' into JDK-8284584-node-idx-duplicate
 - Move

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8164/files
  - new: https://git.openjdk.java.net/jdk/pull/8164/files/21cea3b2..980ba3bf

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8164&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8164&range=00-01

  Stats: 141377 lines in 1379 files changed: 100676 ins; 6905 del; 33796 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