Fix type inference

Fixes oss-fuzz #48908

Author: dstogov

Zend/Optimizer/zend_inference.c

@@ -1936,6 +1936,9 @@ static uint32_t assign_dim_array_result_type(
 			value_type |= MAY_BE_NULL;
 		}
 		if (dim_op_type == IS_UNUSED) {
+			if (arr_type & (MAY_BE_UNDEF|MAY_BE_NULL|MAY_BE_FALSE)) {
+				tmp |= MAY_BE_ARRAY_PACKED;
+			}
 			tmp |= MAY_BE_HASH_ONLY(arr_type) ? MAY_BE_ARRAY_NUMERIC_HASH : MAY_BE_ARRAY_KEY_LONG;
 		} else {
 			if (dim_type & (MAY_BE_LONG|MAY_BE_FALSE|MAY_BE_TRUE|MAY_BE_RESOURCE|MAY_BE_DOUBLE)) {

ext/opcache/tests/opt/inference_013.phpt

@@ -0,0 +1,19 @@
+--TEST--
+Type inference 013: ASSIGN_DIM
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+opcache.optimization_level=-1
+--FILE--
+<?php
+function y(){
+    for(;;){
+        $x[] &= y;
+        $x = false;
+        $x[""] = y;
+    }
+}
+?>
+DONE
+--EXPECT--
+DONE