RFR: 8314106: C2: assert(is_valid()) failed: must be valid after JDK-8305636

[Christian Hagedorn]

In the failing test case, we are unswitching a loop for which we've already removed Parse Predicates with `Compile::cleanup_parse_predicates()`. We are wrongly checking if a predicate block is non-empty (i.e. find the Parse **or** Runtime Predicates) instead of only checking if we find the Parse Predicate:
https://github.com/openjdk/jdk/blob/23fe2ece586d3ed750e905e1b71a2cd1da91f335/src/hotspot/share/opto/loopPredicate.cpp#L448-L453

In the test case, we have a predicate block that contains Runtime Predicates from Loop Predication but no Parse Predicate anymore. Therefore, when trying to clone the non-existing Parse Predicate, we fail with the assertion because we do not have a valid Parse Predicate.

The fix is to only clone a Parse Predicate and the Assertion Predicates for a predicate block if the Parse Predicate is actually there. This is not entirely correct because we would also need to clone Assertion Predicates in the absence of Parse Predicates. But this was already wrong before JDK-8305636:
https://github.com/openjdk/jdk/blob/a38fdaf18dfeeb23775516d1986c720190ba9fc2/src/hotspot/share/opto/loopPredicate.cpp#L598-L612

This will only be fixed with the complete fix ([JDK-8288981](https://bugs.openjdk.org/browse/JDK-8288981)). The proposed fix here just reverts back to the old behavior before JDK-8305636.

Thanks,
Christian

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Commit messages:
 - 8314106: C2: assert(is_valid()) failed: must be valid after JDK-8305636

Changes: https://git.openjdk.org/jdk/pull/15225/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15225&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314106
  Stats: 68 lines in 2 files changed: 66 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15225.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15225/head:pull/15225

PR: https://git.openjdk.org/jdk/pull/15225