nv50/ir: Initialize Graph::Node member tag.

Fix defect reported by Coverity Scan.

Uninitialized scalar field (UNINIT_CTOR)
uninit_member: Non-static class member tag is not initialized in
this constructor nor in any functions that it calls.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10563>

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

@@ -86,7 +86,8 @@ const char *Graph::Edge::typeStr() const
 Graph::Node::Node(void *priv) : data(priv),
                                 in(0), out(0), graph(0),
                                 visited(0),
-                                inCount(0), outCount(0)
+                                inCount(0), outCount(0),
+                                tag(0)
 {
    // nothing to do
 }