RFR: 8357568: IGV: Show NULL and numbers up to 4 characters in "Condense graph" filter

[Christian Hagedorn]

When using the "Condense graph" filter in IGV, it would be useful to show `NULL` and numbers wider than 2 characters instead of `P` and `I/L` (fallback for larger numbers), respectively. There is a comment in `idealGrapPrinter.cpp` which says that maximally 2 chars are allowed for numbers:
https://github.com/openjdk/jdk/blob/428d33ef3ca0af34d8f164fe9d9b722e81e866a7/src/hotspot/share/opto/idealGraphPrinter.cpp#L646

But we already allow larger entries today:
![image](https://github.com/user-attachments/assets/e90d0518-148f-4a33-a9e8-0bdca14aa017)

I there propose to use `NULL` and allow up to 4 characters for numbers which could be a good trade-off between shortness and expressiveness. This allows us to quickly see null checks and larger constants.

Without patch:
![image](https://github.com/user-attachments/assets/e450f1d2-503c-4b84-8137-25892f8ab7f9)


With patch:
![image](https://github.com/user-attachments/assets/81371a53-be7e-4acd-afbf-e5613e96815a)

Thanks,
Christian

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Commit messages:
 - 8357568: IGV: Show NULL and numbers up to 4 characters in "Condense graph" filter

Changes: https://git.openjdk.org/jdk/pull/25393/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25393&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357568
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/25393.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25393/head:pull/25393

PR: https://git.openjdk.org/jdk/pull/25393