//	  CMPQ ...
//	  JLT loop
func loopRotate(f *Func) {
	loopnest := f.loopnest()
	if loopnest.hasIrreducible {
		return
	}
	if len(loopnest.loops) == 0 {
		return
	}

	idToIdx := f.Cache.allocIntSlice(f.NumBlocks())
	defer f.Cache.freeIntSlice(idToIdx)
	for i, b := range f.Blocks {
		idToIdx[b.ID] = i
	}

	// Set of blocks we're moving, by ID.
	move := map[ID]struct{}{}