RFR: 8257190: simplify PhaseIdealLoop constructors

[Xin Liu]

8257190: simplify PhaseIdealLoop constructors

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Commit messages:
 - 8257190: simplify PhaseIdealLoop constructors

Changes: https://git.openjdk.java.net/jdk/pull/1473/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1473&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257190
  Stats: 19 lines in 1 file changed: 4 ins; 11 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1473.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1473/head:pull/1473

PR: https://git.openjdk.java.net/jdk/pull/1473