RFR: 8298301: C2: assert(main_cmp->in(2)->Opcode() == Op_Opaque1) failed: main loop has no opaque node?

[Christian Hagedorn]

This starts to show up in our CI in various tests after [JDK-8269820](https://bugs.openjdk.org/browse/JDK-8269820) which added a new `OpaqueZeroTripGuardNode` but forgot to update an assert which still checks for `Opaque1` instead of `OpaqueZeroTripGuard`. I've fixed that with this patch.

Currently running tests:
- tier1-4

Thanks,
Christian

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Commit messages:
 - Update comment
 - C2: assert(main_cmp->in(2)->Opcode() == Op_Opaque1) failed: main loop has no opaque node?

Changes: https://git.openjdk.org/jdk/pull/11567/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11567&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298301
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11567.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11567/head:pull/11567

PR: https://git.openjdk.org/jdk/pull/11567