RFR: 8257498: Remove useless skeleton predicates [v4]

[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:

  Revert get_skeleton_predicates again

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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/5b716bc5..84723004

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2075&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2075&range=02-03

  Stats: 26 lines in 3 files changed: 7 ins; 13 del; 6 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