shader: let structurization own rollback on failure

src/graphics/shader/recompiler/frontend/cfg/ShaderCFG.h

@@ -110,6 +110,8 @@ struct Graph {
 };
 
 Graph       BuildGraph(const Decoder::Program& program);
+// Commits structured control flow on success; preserves the original graph with
+// failure diagnostics on failure. failure_block is an original block ID or UINT32_MAX.
 bool        Structurize(Graph& graph);
 std::string BranchConditionToString(BranchCondition condition);
 std::string FailureKindToString(FailureKind kind);