Limit the number of relations registered per basic block.

Limit the number of relations registered per basic block.

In pathological cases, the number of transitive relations being added is
potentially quadratic.  Lookups for relations in a block is linear in
nature, so simply limit the number of relations to some reasonable number.

	PR tree-optimization/104038
	* doc/invoke.texi (relation-block-limit): New.
	* params.opt (relation-block-limit): New.
	* value-relation.cc (dom_oracle::register_relation): Check for NULL
	record before invoking transitive registery.
	(dom_oracle::set_one_relation): Check limit before creating record.
	(dom_oracle::register_transitives): Stop when no record created.
	* value-relation.h (relation_chain_head::m_num_relations): New.