Check for equivalence after merging relations.

Check for equivalence after merging relations.

When registering a relation, we need to merge with any existing relation
before checking if it was an equivalence... otherwise it was not being
handled properly.

	gcc/
	PR tree-optimization/105458
	* value-relation.cc (path_oracle::register_relation): Merge, then check
	for equivalence.

	gcc/testsuite/
	* gcc.dg/pr105458.c: New.