RFR: 8371581: C2: PhaseCCP should reach fixpoint by revisiting deeply-Value-d nodes [v3]

[Aleksey Shipilev]

> I started this as investigation into one rare/intermittent CTW failure that I get with [JDK-8360557](https://bugs.openjdk.org/browse/JDK-8360557). The bug seems to reproduce on a very specific JAR with a very specific random seed, so no easy regression test.
> 
> At this point I believe we found that PhaseCCP does not reach the fix point for a peculiar reason: `LoadN` that looks deeply into the graph is not revisited and thus misses the chance to update its type. There is an exception for loads in `verify_Value_for`, but it seems to only apply to constants, and does not apply to `LoadN` in question. Revisiting `LoadN` shows that updating the types downstream performs type widenings (= current types are too narrow), which AFAICS says that this unsound analysis can lead to miscompilation. See more debugging breadcrumbs in the bug.
> 
> It looks like we can reach the fixpoint by recording the nodes we need to revisit and doing another CCP round. This also makes CCP verification stricter: we effectively move 2 exceptional cases recorded in `verify_Value_for` into the analysis itself.
> 
> Testing shows there are no ill effects on correctness doing this. But I would appreciate someone more savvy in this code to sanity check all of this.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, CTW reproducer no longer fails
>  - [x] Linux x86_64 server fastdebug, `all` tests pass
>  - [x] Linux x86_64 server fastdebug, Maven Central CTW passes (!)

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  More comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28288/files
  - new: https://git.openjdk.org/jdk/pull/28288/files/adcfa567..82cd8dae

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28288&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28288&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28288.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28288/head:pull/28288

PR: https://git.openjdk.org/jdk/pull/28288