Fix AOT ?? compile lowering for superglobals (partial #148)

[PurHur]

Summary

• Skip duplicate ArrayDimFetch before ?? when php-cfg emits a ConstFetch (e.g. default literal) between fetch and Coalesce (findsCoalesceUsingFetchAsLeft).
• Defer leftBlock scope inheritance until after ISSET is lowered (avoids slot collision with the isset temp).
• Do not call freeDeadVariables on the coalesce branch block before LLVM branch lowering (merge target was freed early).
• Refresh endBlock scope after coalesce branches are wired.
• Pass dim operand into dimFetch for compile-time string keys; expose IssetHelper::literalStringKey.

Test output (Docker php-compiler:22.04-dev, tar pipe)

script/install-llvm9.sh
export PATH="$PWD/.llvm/bin:$PATH"
export PHP_COMPILER_LLVM_PATH="$PWD/.llvm"
vendor/bin/phpunit --group aot-link --exclude-group serve

Tests: 118, Assertions: 645, Failures: 1.

Remaining failure (blocks merge for full #148)

1) PHPCompiler\AotTest::testCases with data set #18 ('AOT: $_GET['name'] ?? default... #148)')
--- Expected
+++ Actual
@@ @@
-'Ada'
+''

/compiler/test/BaseTest.php:207
/compiler/test/aot/AotTest.php:178

Repro:

printf '%s\n' '<?php' '$name = $_GET["name"] ?? "Guest";' 'echo $name, "\n";' > /tmp/coalesce.php
php bin/compile.php -q name=Ada -o /tmp/coalesce /tmp/coalesce.php
QUERY_STRING=name=Ada /tmp/coalesce   # prints newline only

Tracked as the separate LLVM merge fix called out in commit e02df36 (AOT/JIT coalesce on runtime superglobals remains a separate LLVM merge fix).

Test plan

• vendor/bin/phpunit --group aot-link --exclude-group serve in Docker (117/118 pass)
• coalesce_get_present.phpt AOT output Ada (still open)
• Full ./script/ci-local.sh in Docker when LLVM merge is fixed

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