RFR: 8321278: C2: Partial peeling fails with assert "last_peel <- first_not_peeled"

[Roland Westrelin]

The assert fails because peeling happens at a single entry
`Region`. That `Region` only has a single input because other inputs
were found unreachable and removed by
`PhaseIdealLoop::Dominators()`. The fix I propose is to have
`PhaseIdealLoop::Dominators()` remove the `Region` and its `Phi`s
entirely in this case.

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Commit messages:
 - fix & test

Changes: https://git.openjdk.org/jdk/pull/18353/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18353&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321278
  Stats: 79 lines in 2 files changed: 78 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18353.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18353/head:pull/18353

PR: https://git.openjdk.org/jdk/pull/18353