c++: fix tourney logic

In r13-3766 I changed the logic at the end of tourney to avoid redundant
comparisons, but the change also meant skipping any less-good matches
between the champ_compared_to_predecessor candidate and champ itself.

This should not be a correctness issue, since we believe that joust is a
partial order.  But it can lead to missed warnings, as in this testcase.

gcc/cp/ChangeLog:

	* call.cc (tourney): Only skip champ_compared_to_predecessor.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wsign-promo1.C: New test.