tree-optimization/114855 - speed up dom_oracle::register_transitives

dom_oracle::register_transitives contains an unbound dominator walk
which for the testcase in PR114855 dominates the profile.  The following
fixes the unbound work done by assigning a constant work budget to the
loop, bounding the number of dominators visited but also the number of
relations processed.  This gets both dom_oracle::register_transitives and
get_immediate_dominator off the profile.

I'll note that we're still doing an unbound dominator walk via
equiv_set in find_equiv_dom at the start of the function and when
we register a relation that also looks up the same way.  At least
for the testcase at hand this isn't an issue.

I've also amended the guard to register_transitives with the
per-basic-block limit for the number of relations registered not
being exhausted.

	PR tree-optimization/114855
	* params.opt (--param transitive-relations-work-bound): New.
	* doc/invoke.texi (--param transitive-relations-work-bound):
	Document.
	* value-relation.cc (dom_oracle::register_transitives):
	Assing an overall work budget, bounding the dominator walk and
	the number of relations processed.
	(dom_oracle::record): Only register_transitives when the
	number of already registered relations does not yet exceed
	the per-BB limit.

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