[jdk25u] RFR: 8370405: C2: mismatched store from MergeStores wrongly scalarized in allocation elimination [v2]

[Aleksey Shipilev]

> Fixes JDK 21 -> JDK 25 regression in C2 that can lead to miscompilation. There is a regression test that captures the failure.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, new regression test fails without the fix, passes with it
>  - [ ] Linux x86_64 server fastdebug, `all`

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Backport 59d23095789bbb6d4e466bcbeb82089b17d78eae

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Changes:
  - all: https://git.openjdk.org/jdk25u/pull/382/files
  - new: https://git.openjdk.org/jdk25u/pull/382/files/2ce40615..0a989a57

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk25u&pr=382&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk25u&pr=382&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk25u/pull/382.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/382/head:pull/382

PR: https://git.openjdk.org/jdk25u/pull/382