RFR: 8370502: C2: segfault while adding node to IGVN worklist [v2]

[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 three additional commits since the last revision:

 - fix test spacing
 - Update src/hotspot/share/opto/macro.cpp
   
   Co-authored-by: Manuel Hässig <manuel at haessig.org>
 - Update src/hotspot/share/opto/macro.cpp
   
   Co-authored-by: Manuel Hässig <manuel at haessig.org>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28432/files
  - new: https://git.openjdk.org/jdk/pull/28432/files/80111b97..e8699d79

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28432&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28432&range=00-01

  Stats: 20 lines in 2 files changed: 1 ins; 1 del; 18 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