nv50/ir: make Graph destructor virtual

Avoid ASan new-delete-type-mismatch when Function::domTree is created as
DominatorTree in Function::convertToSSA but destroyed only as base
Graph in ~Function.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/codegen/nv50_ir_graph.h

@@ -147,7 +147,7 @@ public:
 
 public:
    Graph();
-   ~Graph(); // does *not* free the nodes (make it an option ?)
+   virtual ~Graph(); // does *not* free the nodes (make it an option ?)
 
    inline Node *getRoot() const { return root; }