RFR: 8335708: C2: Compile::verify_graph_edges must start at root and safepoints, just like CCP traversal [v2]

[Marc Chevalier]

> In CCP, we transform the nodes going up (toward inputs) starting from root and safepoints because infinite loops can be reachable from the root, but not co-reachable from the root, that is one can follow def-use from root to the loop, but not the use-def from root to loop. For more details, see:
> https://github.com/openjdk/jdk/blob/4cf63160ad575d49dbe70f128cd36aba22b8f2ff/src/hotspot/share/opto/phaseX.cpp#L2063-L2070
> 
> Since we are specifically marking nodes as useful if they are above a safepoint, the check that no dead nodes must be there anymore must also consider nodes above a safepoint as alive: the same criterion must apply. We should nevertheless not start from a safepoint killed by CCP.
> 
> About the test, I use this trick found in `TestInfiniteLoopCCP` because I indeed need a really infinite loop, but I want a terminating test. The crash is not deterministic, as it needs StressIGVN, so I did a bit of stats. Using a little helper script, on 100 runs, 69 runs fail as in the JBS ticket and 31 are successful (so 0 fail in another way). After the fix, I find 100 successes.
> 
> And thanks to @eme64  who extracted such a concise reproducer.

Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:

  various fixes

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/23977/files
  - new: https://git.openjdk.org/jdk/pull/23977/files/a07dc231..6f8fce6e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23977&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23977&range=00-01

  Stats: 157 lines in 3 files changed: 81 ins; 69 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/23977.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23977/head:pull/23977

PR: https://git.openjdk.org/jdk/pull/23977