RFR: 8257498: Remove useless skeleton predicates

[Christian Hagedorn]

This enhancement removes useless skeleton predicates in the same way as we already remove normal useless predicates in `PhaseIdealLoop::eliminate_useless_predicates()`.

Thanks,
Christian

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Commit messages:
 - 8257498: Remove useless skeleton predicates

Changes: https://git.openjdk.java.net/jdk/pull/2075/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2075&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257498
  Stats: 92 lines in 5 files changed: 58 ins; 16 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2075.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2075/head:pull/2075

PR: https://git.openjdk.java.net/jdk/pull/2075