RFR: 8343206: Final graph reshaping should not compress abstract or interface class pointers

[Tobias Hartmann]

@fisk found this problematic optimization in final graph reshaping where we would convert a `CmpP` into a `CmpN` by converting a constant class pointer operand to a narrow class pointer. After  [JDK-8338526](https://bugs.openjdk.org/browse/JDK-8338526), this is not valid if the class pointer refers to an interface or abstract class.

I think it's not an issue in current code though. The only way we can get a dynamic narrow class is when loading from an object at `oopDesc::klass_offset_in_bytes()`:
https://github.com/openjdk/jdk/blob/7131f053b0d26b62cbf0d8376ec117d6e8d79f9e/src/hotspot/share/opto/type.cpp#L3521-L3522

Comparisons of such loads with a constant class pointer of interface or abstract class type are always folded during GVN:
https://github.com/openjdk/jdk/blob/7131f053b0d26b62cbf0d8376ec117d6e8d79f9e/src/hotspot/share/opto/subnode.cpp#L1164-L1171

And therefore, the code in `Compile::final_graph_reshaping_main_switch` will never trigger.

I added a corresponding assert and bailout in product to be on the safe side. The assert never triggered in my testing.

Thanks,
Tobias

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Commit messages:
 - 8343206: Final graph reshaping should not compress abstract or interface class pointers

Changes: https://git.openjdk.org/jdk/pull/21784/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21784&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343206
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21784.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21784/head:pull/21784

PR: https://git.openjdk.org/jdk/pull/21784