func (s *state) updateUnsetPredPos(b *ssa.Block) {
	if b.Pos == src.NoXPos {
		s.Fatalf("Block %s should have a position", b)
	}
	bestPos := src.NoXPos
	for _, e := range b.Preds {
		p := e.Block()
		if !p.LackingPos() {
			continue
		}
		if bestPos == src.NoXPos {
			bestPos = b.Pos
			for _, v := range b.Values {
				if v.LackingPos() {
					continue
				}