Use-after-free for ??= due to incorrect live-range calculation

Fixes GHSA-rwp7-7vc6-8477

Author: iluuu1994

Zend/tests/ghsa-rwp7-7vc6-8477_001.phpt

@@ -0,0 +1,26 @@
+--TEST--
+GHSA-rwp7-7vc6-8477: Use-after-free for ??= due to incorrect live-range calculation
+--FILE--
+<?php
+
+class Foo {
+    public function foo() {
+        return $this;
+    }
+
+    public function __set($name, $value) {
+        throw new Exception('Hello');
+    }
+}
+
+$foo = new Foo();
+
+try {
+    $foo->foo()->baz ??= 1;
+} catch (Exception $e) {
+    echo $e->getMessage();
+}
+
+?>
+--EXPECT--
+Hello

Zend/tests/ghsa-rwp7-7vc6-8477_002.phpt

@@ -0,0 +1,24 @@
+--TEST--
+GHSA-rwp7-7vc6-8477: Use-after-free for ??= due to incorrect live-range calculation
+--FILE--
+<?php
+
+class Foo {
+    public int $prop;
+
+    public function foo() {
+        return $this;
+    }
+}
+
+$foo = new Foo();
+
+try {
+    $foo->foo()->prop ??= 'foo';
+} catch (Error $e) {
+    echo $e->getMessage();
+}
+
+?>
+--EXPECT--
+Cannot assign string to property Foo::$prop of type int

Zend/tests/ghsa-rwp7-7vc6-8477_003.phpt

@@ -0,0 +1,22 @@
+--TEST--
+GHSA-rwp7-7vc6-8477: Use-after-free for ??= due to incorrect live-range calculation
+--FILE--
+<?php
+
+class Foo {
+    public int $prop;
+}
+
+function newFoo() {
+    return new Foo();
+}
+
+try {
+    newFoo()->prop ??= 'foo';
+} catch (Error $e) {
+    echo $e->getMessage();
+}
+
+?>
+--EXPECT--
+Cannot assign string to property Foo::$prop of type int

Zend/zend_opcode.c

@@ -940,6 +940,14 @@ static void zend_calc_live_ranges(
 		opnum--;
 		opline--;
 
+		/* SEPARATE always redeclares its op1. For the purposes of live-ranges,
+		 * its declaration is irrelevant. Don't terminate the current live-range
+		 * to avoid breaking special handling of COPY_TMP. */
+		if (opline->opcode == ZEND_SEPARATE) {
+			ZEND_ASSERT(opline->op1.var == opline->result.var);
+			continue;
+		}
+
 		if ((opline->result_type & (IS_TMP_VAR|IS_VAR)) && !is_fake_def(opline)) {
 			uint32_t var_num = EX_VAR_TO_NUM(opline->result.var) - var_offset;
 			/* Defs without uses can occur for two reasons: Either because the result is