Fix AOT superglobal isset and coalesce lowering (partial #99, #148)

[PurHur]

Summary

• Fix superglobal isset() in LLVM/AOT to use runtime __hashtable__offsetIsSetStringKey instead of baking compile-time false when QUERY_STRING is only known at run time.
• Fix ?? lowering when php-cfg emits Assign after Coalesce: write into the assignee slot, reuse the same $_GET slot for isset and fetch, and inherit scope after isset operands are compiled.
• Fix coalesce branch wiring in JIT: merge branches jump from branch tail blocks (after fetch/assign), not entry blocks that skipped the fetch IR.

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

composer install && ./script/apply-patches.sh
export PATH=/opt/llvm9/bin:$PATH
vendor/bin/phpunit --group aot-link --exclude-group serve

Tests: 117, Assertions: 640, Failures: 1.

Before (master): 2 failures (coalesce_get_present, superglobal_get_assign munmap).

After: 1 failure remaining.

Remaining failure (needs follow-up)

vendor/bin/phpunit --group aot-link --filter coalesce_get_present

Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'Ada'
+''

Repro:

vendor/bin/phpunit test/aot/AotTest.php --filter 'coalesce_get_present'
# or compile + run:
php bin/compile.php -q name=Ada -o /tmp/coal - <<'PHP'
<?php
$name = $_GET['name'] ?? 'Guest';
echo $name, "\n";
PHP
QUERY_STRING=name=Ada /tmp/coal   # prints blank line only

VM path works with the same sources (bin/vm.php prints Ada). Issue is LLVM merge/echo of the coalesce result, not the compiler CFG.

Test plan

• vendor/bin/phpunit --group aot-link --exclude-group serve in Docker (116/117 pass)
• coalesce_get_present AOT output Ada / Guest (tracked above)
• Full ./script/ci-local.sh in Docker when merging

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