[jdk20] RFR: 8298520: C2: assert(found_opaque == res) failed: wrong pattern

[Roland Westrelin]

The assert fires because CountedLoopNode::is_canonical_loop_entry()
finds an Opaque1 where it expects an OpaqueZeroTripGuard. That Opaque1
guards the CountedLoopEnd of the pre loop: is_canonical_loop_entry()
should have stopped at the zero trip guard but walked past it. The
reason for that is the call for skip_predicates(): when
CountedLoopNode::skip_predicates_from_entry() encounters an If node
that has lost a projection, it assumes it's a predicate and moves to
the next one. In this case, the zero trip guard only has one
projection (the one that connects it through predicates to the loop
head), because igvn is in the process of updating a dead part of the
graph.

To fix this, I propose that:

1- CountedLoopNode::is_canonical_loop_entry() fails when after
predicates it's at a CountedLoopEnd.

2- CountedLoopNode::skip_predicates_from_entry() stops at a zero trip
guard (identified by a condition that uses a OpaqueZeroTripGuard)

Either 1- or 2- are good enough to fix this particular graph but I
propose doing both as making this logic more robust feels safer.

I don't provide a test case as this seems to only happen when igvn
processes a dying subgraph in a very specific order.

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

Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk20/pull/24/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20&pr=24&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298520
  Stats: 21 lines in 2 files changed: 19 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk20/pull/24.diff
  Fetch: git fetch https://git.openjdk.org/jdk20 pull/24/head:pull/24

PR: https://git.openjdk.org/jdk20/pull/24