Maintain complex constraint vector order during PTA solving

Maintain complex constraint vector order during PTA solving

There's a FIXME comment in the PTA constraint solver that the vector
of complex constraints can get unsorted which can lead to duplicate
entries piling up during node unification.  The following fixes this
with the assumption that delayed updates to constraints are uncommon
(otherwise re-sorting the whole vector would be more efficient).

	* tree-ssa-structalias.cc (constraint_equal): Take const
	reference to constraints.
	(constraint_vec_find): Similar.
	(solve_graph): Keep constraint vector sorted and verify
	sorting with checking.