RFR: 8257498: Remove useless skeleton predicates [v3]

[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

Christian Hagedorn has updated the pull request incrementally with one additional commit since the last revision:

  Fix removal from global list and update get_skeleton_predicates

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2075/files
  - new: https://git.openjdk.java.net/jdk/pull/2075/files/0ed11ba9..5b716bc5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2075&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2075&range=01-02

  Stats: 39 lines in 6 files changed: 23 ins; 9 del; 7 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