RFR: 8328480: C2: SubTypeCheckNode in checkcast should use the klass constant of a unique concrete sub class

[Christian Hagedorn]

While working on a [Valhalla bug](https://bugs.openjdk.org/browse/JDK-8321734), I've noticed that a `SubTypeCheckNode` for a `checkcast` does not take a unique concrete sub class `X` of an abstract class `A` as klass constant in the sub type check. Instead, it uses the abstract klass constant:


abstract class A {}
class X extends A {}

A x = (A)object; // Emits SubTypeCheckNode(object, A), but could have used X instead of A.

However, the `CheckCastPP` result already uses the improved instance type ptr `X` (i.e. `toop` which was improved from `A` by calling `try_improve()` to get the unique concrete sub class):
https://github.com/openjdk/jdk/blob/614db2ea9e10346475eef34629eab54878aa482d/src/hotspot/share/opto/graphKit.cpp#L3257-L3261
https://github.com/openjdk/jdk/blob/614db2ea9e10346475eef34629eab54878aa482d/src/hotspot/share/opto/graphKit.cpp#L3363

We should also plug in a unique concrete sub class constant in the `SubTypeCheckNode` which could be beneficial to fold away redundant sub type checks (see test cases).

This fix is required to completely fix the bug in Valhalla (this is only one of the broken cases). In Valhalla, the graph ends up being broken because a `CheckCastPP` node is folded because of an impossible type but the `SubTypeCheckNode` is not due to not using the improved unique concrete sub class constant for the `checkcast`. I don't think that there is currently a bug in mainline because of this limitation - it just blocks some optimizations. I'm therefore upstreaming this fix to mainline since it can be beneficial to have this fix here as well (see test cases).

Thanks,
Christian

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Commit messages:
 - 8328480: C2: SubTypeCheckNode in checkcast should use the klass constant of a unique concrete sub class

Changes: https://git.openjdk.org/jdk/pull/18515/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18515&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328480
  Stats: 85 lines in 2 files changed: 78 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/18515.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18515/head:pull/18515

PR: https://git.openjdk.org/jdk/pull/18515