removed hidden union tag in release modes

Author: emekoi

src/analyze.cpp

@@ -2886,7 +2886,7 @@ static Error resolve_union_zero_bits(CodeGen *g, ZigType *union_type) {
     union_type->data.unionation.have_explicit_tag_type = decl_node->data.container_decl.auto_enum ||
         enum_type_node != nullptr;
     bool auto_layout = (union_type->data.unionation.layout == ContainerLayoutAuto);
-    bool want_safety = (field_count >= 2) && (auto_layout || enum_type_node != nullptr);
+    bool want_safety = (field_count >= 2) && (auto_layout || enum_type_node != nullptr) && !(g->build_mode == BuildModeFastRelease || g->build_mode == BuildModeSmallRelease);
     ZigType *tag_type;
     bool create_enum_type = decl_node->data.container_decl.auto_enum || (enum_type_node == nullptr && want_safety);
     bool *covered_enum_fields;