JIT: fix global/foreach/try assign lowering for AOT spine (#16828)

[PurHur]

Summary

• Prescan global $name imports across the whole function CFG before lowering try-merge blocks (foreach valid phi could compile before DECLARE_GLOBAL).
• Teach assignOperandValue / assignOperand to handle native-scalar and string KIND_VALUE phi operands instead of throwing.
• Harden TYPE_ASSIGN / TYPE_CONCAT when alias/dest operands are null (literal-RHS peephole).

Closes #16828 (assign-guard blocker). Full spine bundle now progresses past Cannot assign to a value / Cannot assignOperandValue to a value and fails later on Forward reference to class constant DateException::class (separate compile issue).

Verification

php bin/compile.php -o /tmp/aot_test test/fixtures/aot/compile-only/global_foreach_try_after_early_return.php
# (no output, exit 0)

./script/docker-exec.sh -- bash -lc './vendor/bin/phpunit test/unit/BootstrapAotJitCompileTest.php --filter testGlobalForeachTryAfterEarlyReturnCompiles'
# OK (1 test, 5 assertions)

php bin/compile.php -o /tmp/spine test/selfhost/compiler_lib_spine_smoke/main.php 2>&1 | tail -1
# Forward reference to class constant DateException::class
# (was: Cannot assign to a value)

Test plan

• Issue repro fixture AOT-compiles
• BootstrapAotJitCompileTest::testGlobalForeachTryAfterEarlyReturnCompiles (Docker/LLVM)
• Spine smoke compile no longer hits assignOperand guard (next blocker documented)

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