RFR: 8370502: C2: segfault while adding node to IGVN worklist [v6]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 11 additional commits since the last revision:

 - Merge branch 'master' into fix-c2-segfault-unlocknode
 - Merge branch 'master' into fix-c2-segfault-unlocknode
 - address comments
 - fix rename
 - rename test file
 - Merge branch 'master' into fix-c2-segfault-unlocknode
 - 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>
 - copyright format fix?
 - ... and 1 more: https://git.openjdk.org/jdk/compare/1574ff53...b5e878c7

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28432/files
  - new: https://git.openjdk.org/jdk/pull/28432/files/21018290..b5e878c7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28432&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28432&range=04-05

  Stats: 25873 lines in 499 files changed: 17932 ins; 6014 del; 1927 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