RFR: 8355230: Crash in fuzzer tests: assert(n != nullptr) failed: must not be null [v2]

[Roland Westrelin]

> During IGVN, `TypeNode::make_paths_from_here_dead()` follows data
> nodes until a `Phi`. The `Region` input for the input that that logic
> goes through to reach the `Phi` is `null` causing the crash. I propose
> simply adding an extra check for that corner case.

Roland Westrelin has updated the pull request incrementally with two additional commits since the last revision:

 - test comment
 - test cleanup

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/25268/files
  - new: https://git.openjdk.org/jdk/pull/25268/files/22058200..1ab90292

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25268&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25268&range=00-01

  Stats: 40 lines in 1 file changed: 4 ins; 20 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/25268.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25268/head:pull/25268

PR: https://git.openjdk.org/jdk/pull/25268