// own trivial phi functions in their own dominance frontier, and this is handled recursively.
func addDFphis(x *Value, h, b *Block, f *Func, defForUses []*Value, newphis map[*Block]rewrite, sdom SparseTree) {
	oldv := defForUses[b.ID]
	if oldv != x { // either a new definition replacing x, or nil if it is proven that there are no uses reachable from b
		return
	}
	idom := f.Idom()
outer: