[release/9.0-staging] [wasm][AOT] fix codegen for small structs on stack (#118416)

Author: github-actions[bot]

src/mono/mono/mini/mini-wasm.c

@@ -798,18 +798,20 @@ mini_wasm_is_scalar_vtype (MonoType *type, MonoType **etype)
 } else if (MONO_TYPE_ISSTRUCT (t)) {
 if (!mini_wasm_is_scalar_vtype (t, etype))
 return FALSE;
-} else if (!((MONO_TYPE_IS_PRIMITIVE (t) || MONO_TYPE_IS_REFERENCE (t) || MONO_TYPE_IS_POINTER (t)))) {
+} else if (!(MONO_TYPE_IS_PRIMITIVE (t) || MONO_TYPE_IS_REFERENCE (t) || MONO_TYPE_IS_POINTER (t))) {
 return FALSE;
 } else {
 if (etype)
 *etype = t;
 }
 }
 
-if (etype) {
-if (!(*etype))
-*etype = mono_get_int32_type ();
+// empty struct
+if (nfields == 0 && etype) {
+*etype = m_class_get_byval_arg (mono_defaults.sbyte_class);
 }
 
+g_assert (!etype || *etype);
+
 return TRUE;
 }