tree-optimization/114855 - slow VRP due to equiv oracle queries

For the testcase in PR114855 VRP takes 320.41s (23%) (after mitigating
backwards threader slowness).  This is mostly due to the bitmap check
in equiv_oracle::find_equiv_dom.  The following turns this bitmap
to tree view, trading the linear search for a O(log N) one which
improves VRP time to 54.54s (5%).

	PR tree-optimization/114855
	* value-relation.cc (equiv_oracle::equiv_oracle): Switch
	m_equiv_set to tree view.