RFR: 8285369: C2: emit reduction flag value in node and loop dumps

[Roberto Castañeda Lozano]

Tris (trivial?) enhancement emits the `LoopNode::HasReductions` flag from `CountedLoopNode::dump_spec()` and `IdealLoopTree::dump_head()`, and `Node::Flag_is_reduction` from `IdealGraphPrinter::print()`. This proved to be useful in the investigation of [JDK-8279622](https://bugs.openjdk.java.net/browse/JDK-8279622).

Tested on linux-x64 (build and hs-tier1).

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Commit messages:
 - Emit reduction flags in node, loop, and IGV graph dumps

Changes: https://git.openjdk.java.net/jdk/pull/8336/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8336&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285369
  Stats: 6 lines in 2 files changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8336.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8336/head:pull/8336

PR: https://git.openjdk.java.net/jdk/pull/8336