Fix invalid opcode for ??= on $GLOBALS

Fixes #81684

Author: iluuu1994

Zend/tests/bug81684.phpt

@@ -0,0 +1,11 @@
+--TEST--
+Bug #81684: ??= on $GLOBALS produces an invalid opcode
+--FILE--
+<?php
+$GLOBALS['x'] ??= 'x'; // Fatal error: Invalid opcode 23/1/0
+var_dump($GLOBALS['x']);
+echo "Done.\n";
+?>
+--EXPECT--
+string(1) "x"
+Done.

Zend/zend_compile.c

@@ -8919,7 +8919,9 @@ static void zend_compile_assign_coalesce(znode *result, zend_ast *ast) /* {{{ */
 
 	/* Reproduce some of the zend_compile_assign() opcode fixup logic here. */
 	opline = &CG(active_op_array)->opcodes[CG(active_op_array)->last-1];
-	switch (var_ast->kind) {
+	/* Treat $GLOBALS['x'] assignment like assignment to variable. */
+	zend_ast_kind kind = is_global_var_fetch(var_ast) ? ZEND_AST_VAR : var_ast->kind;
+	switch (kind) {
 		case ZEND_AST_VAR:
 			zend_emit_op_tmp(&assign_node, ZEND_ASSIGN, &var_node_w, &default_node);
 			break;