mesa: Fix test for optimistic coloring being necessary.

i965 and radeon use ra_set_node_reg() to force payload registers to
specific registers while exposing those registers to the allocator still.
We were treating those register nodes as unsuccessfully allocated in the
ra_simplify() step, leading to walking the registers again to do
optimistic coloring even if there was nothing left ot do.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/program/register_allocate.c

@@ -437,7 +437,7 @@ ra_simplify(struct ra_graph *g)
    }
 
    for (i = 0; i < g->count; i++) {
-      if (!g->nodes[i].in_stack)
+      if (!g->nodes[i].in_stack && g->nodes[i].reg == -1)
 	 return GL_FALSE;
    }