RFR: 8370502: C2: segfault while adding node to IGVN worklist

[Kerem Kat]

Do not try to replace `fallthrough_memproj` when it is null, fixes crash.

Test case is simplified from the ticket. Verified that the case crashes without the fix.

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Commit messages:
 - copyright format fix?
 - 8370502: C2: segfault while adding node to IGVN worklist

Changes: https://git.openjdk.org/jdk/pull/28432/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28432&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370502
  Stats: 68 lines in 2 files changed: 56 ins; 5 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/28432.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28432/head:pull/28432

PR: https://git.openjdk.org/jdk/pull/28432