RFR: 8335708: C2: assert(!dead_nodes) failed: using nodes must be reachable from root

[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.

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Commit messages:
 - Don't start on removed safepoints.
 - Start detection of useful nodes also at safepoints

Changes: https://git.openjdk.org/jdk/pull/23977/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23977&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335708
  Stats: 98 lines in 5 files changed: 86 ins; 0 del; 12 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