RFR: 8283541: Add Statical counters and some comments in PhaseStringOpts

[Xin Liu]

8283541: Add Statical counters and some comments in PhaseStringOpts

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Commit messages:
 - 8283541: Add Statical counters and some comments in PhaseStringOpts

Changes: https://git.openjdk.java.net/jdk/pull/7933/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7933&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283541
  Stats: 97 lines in 3 files changed: 48 ins; 41 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7933.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7933/head:pull/7933

PR: https://git.openjdk.java.net/jdk/pull/7933