[jdk21u] RFR: 8313248: C2: setScopedValueCache intrinsic exposes nullptr pre-values to store barriers

[Aleksey Shipilev]

Clean backport to fix Loom bug reproducing at least with Shenandoah, but also maybe with G1.

Additional testing:
 - [x] macos-aarch64-server-fastdebug, original reproducer fails without the patch, passes with it

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Commit messages:
 - Backport e8a37b90db8dca4dc3653970b2d66d2faf8ef452

Changes: https://git.openjdk.org/jdk21u/pull/50/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=50&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313248
  Stats: 22 lines in 2 files changed: 10 ins; 8 del; 4 mod
  Patch: https://git.openjdk.org/jdk21u/pull/50.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/50/head:pull/50

PR: https://git.openjdk.org/jdk21u/pull/50