php-in-php: delete dead array_merge LLVM from ArrayBuiltinHelper (#10183)

[PurHur]

Summary

Follow-up slice of #10183 / #1492: array_merge() and array_merge_recursive() JIT/AOT already route through ArrayMergeRuntime + ArrayMergeRecursiveRuntime → ArrayMergeJitHelper / ArrayMergeRecursiveJitHelper PHP (SSOT: VmArray / HashTable::mergeRecursiveCopy()). The old LLVM paths in ArrayBuiltinHelper were unreachable dead code.

LLVM shrink

Removed	Lines
ArrayBuiltinHelper::merge() + mergeSingleArgumentCopy() LLVM	~160
ArrayBuiltinHelper::mergeRecursive() + overlay LLVM subtree	~300
ext/standard/JitArrayMergeRecursive.php shim	18
Net ArrayBuiltinHelper.php	−468

Kept appendValueEntryToPacked() / storeValueEntryAtIndexWithHashtable() (still used by copyInto()).

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit --filter ArrayMergeRecursiveRuntimeShrinkTest   # OK (3 tests)
vendor/bin/phpunit --filter ArrayMergeRuntimeShrinkTest            # OK (2 tests)
vendor/bin/phpunit test/compliance/VMTest.php --filter array_merge_recursive  # OK (7 tests)
make bootstrap-selfhost-vm-driver-execute-probe                    # OK
php script/bootstrap-inventory.php && php script/check-selfhost-spine-coverage-sync.php
'

Note: 2 AotTest array_merge_recursive cases fail on master too (HashTable::mergerecursivecopy() AOT naming) — pre-existing, not introduced here.

Closes #10183 (dead-code slice; epic tracking continues under #1492).

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