util/ra: Only update best_optimistic_node if !progress

This shaves about half a second off the 30 second compile time of one of
the compute shaders in Aztec ruins.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/util/register_allocate.c

@@ -524,7 +524,11 @@ ra_simplify(struct ra_graph *g)
             g->stack_count++;
             BITSET_SET(g->in_stack, i);
             progress = true;
-         } else {
+         } else if (!progress) {
+            /* We only need to do this if we haven't made progress.  If we
+             * have made progress, we'll throw the data away and loop again
+             * anyway.
+             */
             unsigned int new_q_total = g->nodes[i].q_total;
             if (new_q_total < lowest_q_total) {
                best_optimistic_node = i;