RFR: 8326638: Crash in PhaseIdealLoop::remix_address_expressions due to unexpected Region instead of Loop

[Tobias Hartmann]

Adds a missing `_head->is_Loop()` check to handle the case when the loop head is not a `LoopNode` but a `RegionNode` because the loop is irreducible:


(rr) p n_loop->_head->dump(1)
  412 IfTrue === 411 [[ 345 ]] #1 !jvms: Test$Class1::Class1_method0 @ bci:286 (line 119)
  339 SafePoint === 336 1 340 1 1 344 290 1 1 1 1 291 1 1 293 294 295 1 320 297 1 298 299 300 301 302 1 303 1 [[ 345 ]] SafePoint !jvms: Test$Class1::Class1_method0 @ bci:307 (line 122)
  345 Region === 345 339 412 [[ 345 456 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 1861 1863 1865 1867 1882 1888 ]] #irreducible !jvms: Test$Class1::Class1_method0 @ bci:138 (line 101)


We then crash when invoking `LoopNode::skip_strip_mined` on a RegionNode or assert in debug. Unfortunately, I was not able to simply the test any further.

This is a regression from [JDK-8303511](https://bugs.openjdk.org/browse/JDK-8303511) in JDK 21 b13

Thanks,
Tobias

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Commit messages:
 - Removed whitespace
 - Missing comment
 - Added test
 - 8326638: Crash in PhaseIdealLoop::remix_address_expressions due to unexpected Region instead of Loop

Changes: https://git.openjdk.org/jdk/pull/18002/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18002&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326638
  Stats: 64 lines in 2 files changed: 61 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18002.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18002/head:pull/18002

PR: https://git.openjdk.org/jdk/pull/18002