RFR: 8347499: C2: Make `PhaseIdealLoop` eliminate more redundant safepoints in loops

[Qizheng Xing]

In `PhaseIdealLoop`, `IdealLoopTree::check_safepts` method checks if any call that is guaranteed to have a safepoint dominates the tail of the loop. In the previous implementation, `check_safepts` would stop if it found a local non-call safepoint. At this time, if there was a call before the safepoint in the dom-path, this safepoint would not be eliminated.

<img width="353" alt="loop-safepoint" src="https://github.com/user-attachments/assets/c220e103-aaba-4e3f-98ac-1ddb6465c309" />

This patch changes the behavior of `check_safepts` to not stop when it finds a non-local safepoint. This makes simple loops with one method call ~3.8% faster (on aarch64).


Benchmark                Mode  Cnt       Score      Error  Units
LoopSafepoint.loopVar    avgt   15  208296.259 ± 1350.409  ns/op   # baseline
LoopSafepoint.loopVar    avgt   15  200692.874 ±  616.770  ns/op   # this patch


Testing: tier1-2 on x86_64 and aarch64.

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Commit messages:
 - Add IR test and microbench.
 - Make `PhaseIdealLoop` eliminate more redundant safepoints in loops.

Changes: https://git.openjdk.org/jdk/pull/23057/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23057&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347499
  Stats: 166 lines in 3 files changed: 162 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23057.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23057/head:pull/23057

PR: https://git.openjdk.org/jdk/pull/23057