private V getImmediateDominator(V v) {
		V dom = dominatorMap.get(v);
		if (isDummy(dom)) {
			return null;
		}
		return dom;
	}
Returns the dominance tree for the given graph, which is tree where each 
node's children are those nodes it *immediately* dominates (a idom b).

@return the dominance tree
Ghidra/Framework/Graph/src/main/java/ghidra/graph/algo/ChkDominanceAlgorithm.java