RFR: 8370502: C2: segfault while adding node to IGVN worklist [v4]

[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.

Kerem Kat has updated the pull request incrementally with one additional commit since the last revision:

  fix rename

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28432/files
  - new: https://git.openjdk.org/jdk/pull/28432/files/d0971b5b..a0f0ecb9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28432&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28432&range=02-03

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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