RFR: 8335220: C2: Missing check for Opaque4 node in EscapeAnalysis [v2]

[Vladimir Kozlov]

> While testing Leyden Early Release the problem was found in C2 EscapeAnalysis, in new code which handles allocations merge. `ConnectionGraph:: can_reduce_check_users()` have incomplete checks which missed case with `Opaque4` node between `If` and `Bool` nodes. As result we hit assert in `ConnectionGraph::specialize_cmp()` when trying to access inputs of `Cmp` node
> 
> Node* curr_cmp = curr_ctrl->in(0)->in(1)->in(1); // true/false -> if -> bool -> cmp
> con = curr_cmp->in(1)->is_Con() ? curr_cmp->in(1) : curr_cmp->in(2);
> 
> 
> This code was introduce for ReduceAllocationMerges optimization [JDK-8316991](https://bugs.openjdk.org/browse/JDK-8316991) in JDK-23. There was followup fix [JDK-8330853](https://bugs.openjdk.org/browse/JDK-8330853) to add more checks but it missed this case.
> 
> I added more strict check in `can_reduce_check_users()` to catch such cases. And added asserts in `specialize_cmp()` to verify expected nodes.
> 
> Tested tier1-3.hs-stress,hs-xcomp

Vladimir Kozlov has updated the pull request incrementally with one additional commit since the last revision:

  Add comment about Opaque4

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19921/files
  - new: https://git.openjdk.org/jdk/pull/19921/files/a0687aee..c06f6ce0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19921&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19921&range=00-01

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19921.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19921/head:pull/19921

PR: https://git.openjdk.org/jdk/pull/19921