RFR: Merge lworld

[Vicente Romero]

Merge branch 'lworld' into lw5_merge_lworld
# Conflicts:
#	test/langtools/tools/javac/diags/examples/ReturnBeforeSuperclassInit.java

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Commit messages:
 - Merge branch 'lworld' into lw5_merge_lworld
 - 8376145: [lworld] Enable SubstitutabilityTest.java with AOTJDK
 - 8376128: [lworld] ProblemList valhalla/valuetypes/SubstitutabilityTest.java with AOTCache again
 - 8368274: [lworld] Unsafe.compareAndExchangeFlatValue() wrongly assumes padding bytes are the same for identical value objects
 - 8375719: [lworld] Move reads of flat fields entirely to the runtime
 - 8374800: [lworld] Add a NULLABLE_NON_ATOMIC_FLAT layout
 - 8374006: [lworld] Improve field accessor methods in InlineTypeNode
 - 8374122: [lworld] compiler/c2/cmove/TestScalarConditionalMoveCmpObj.java fails with ir mismatches when preview enabled, post jdk-27+3
 - Merge
 - 8354853: Clean up x86 registers after 32-bit x86 removal
 - ... and 250 more: https://git.openjdk.org/valhalla/compare/2041a5a9...50a5d2ba

The webrevs contain the adjustments done while merging with regards to each parent branch:
 - lw5: https://webrevs.openjdk.org/?repo=valhalla&pr=1964&range=00.0
 - lworld: https://webrevs.openjdk.org/?repo=valhalla&pr=1964&range=00.1

Changes: https://git.openjdk.org/valhalla/pull/1964/files
  Stats: 65496 lines in 3122 files changed: 31228 ins; 13452 del; 20816 mod
  Patch: https://git.openjdk.org/valhalla/pull/1964.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1964/head:pull/1964

PR: https://git.openjdk.org/valhalla/pull/1964