RFR: 8321278: C2: Partial peeling fails with assert "last_peel <- first_not_peeled" [v4]

[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.

Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:

  comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18353/files
  - new: https://git.openjdk.org/jdk/pull/18353/files/d201aee2..8fbb6f96

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18353&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18353&range=02-03

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 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