RFR: 8282056: Clean up com.sun.tools.javac.util.GraphUtils

[Pavel Rappo]

Please review this trivial cleanup.

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Commit messages:
 - Fix code
 - Fix comments

Changes: https://git.openjdk.java.net/jdk/pull/7520/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7520&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282056
  Stats: 12 lines in 2 files changed: 1 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7520.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7520/head:pull/7520

PR: https://git.openjdk.java.net/jdk/pull/7520