RFR: 8251464: make Node::dump(int depth) support indent

[Xin Liu]

Node::dump(depth) indents 2 whitespaces for each level.
The function isnot on until the depth of the ideal graph isnot greater
than PrintIdealIndentThreshold (0 by default).

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Commit messages:
 - 8251464: make Node::dump(int depth) support indent

Changes: https://git.openjdk.java.net/jdk/pull/371/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=371&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251464
  Stats: 49 lines in 5 files changed: 26 ins; 0 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/371.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/371/head:pull/371

PR: https://git.openjdk.java.net/jdk/pull/371