c++: non-dep reversed <=> returning int [PR123601]

c++: non-dep reversed <=> returning int [PR123601]

The code path for non-dependent operator expressions rewritten from a
<=> returning a non-class type (added in r16-3727-gf2fddc4b84a843) is
also reached for a reversed such <=> expression with the form
0 @ (y <=> x) where the @ is <=>, so we need to relax the relevant
assert accordingly.

	PR c++/123601

gcc/cp/ChangeLog:

	* tree.cc (build_min_non_dep_op_overload): Relax
	COMPARISON_CLASS_P assert to accept SPACESHIP_EXPR too.

gcc/testsuite/ChangeLog:

	* g++.dg/lookup/operator-9.C: New test.