RFR: 8347499: C2: Make `PhaseIdealLoop` eliminate more redundant safepoints in loops [v3]

[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.

Qizheng Xing has updated the pull request incrementally with one additional commit since the last revision:

  Improve documentation comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23057/files
  - new: https://git.openjdk.org/jdk/pull/23057/files/56983ed5..1a216046

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23057&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23057&range=01-02

  Stats: 20 lines in 1 file changed: 8 ins; 0 del; 12 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