soften heuristic of 316b57

Instead of systematically spilling any
temp live in an exit branch but not in
the part of the loop already processed,
only spill when it is already known to
have been spilled.

spill.c

@@ -280,6 +280,19 @@ dopm(Blk *b, Ins *i, BSet *v)
 	return i;
 }
 
+static void
+merge(BSet *u, Blk *bu, BSet *v, Blk *bv)
+{
+	int t;
+
+	if (bu->loop <= bv->loop)
+		bsunion(u, v);
+	else
+		for (t=0; bsiter(v, &t); t++)
+			if (tmp[t].slot == -1)
+				bsset(u, t);
+}
+
 /* spill code insertion
  * requires spill costs, rpo, liveness
  *
@@ -364,12 +377,10 @@ spill(Fn *fn)
 			 * in the middle of loops */
 			bszero(v);
 			liveon(w, b, s1);
-			if (s1->loop >= b->loop)
-				bsunion(v, w);
+			merge(v, b, w, s1);
 			if (s2) {
 				liveon(u, b, s2);
-				if (s2->loop >= b->loop)
-					bsunion(v, u);
+				merge(v, b, u, s2);
 				bsinter(w, u);
 			}
 			limit2(v, 0, 0, w);